Electronics, Lighting, & Gadgets - Cyclometer or Handheld GPS??

Im looking to get something for my bike that at least tells me my altidude along with speed , average speed distance blah blah... but Im wondering if a handheld GPS unit would be better. I like the idea of seeing on a display where Ive been and having a trail to look back on as well. But do GPS units display speed?

But do GPS units display speed?

Yep

Keep in mind that a GPS works by using satellites. I love mine, but every once in a while, it'll blip and say my max speed was 400 MPH or something like that. In general though, it works well. It doesn't work as well if you use it amongst tall buildings and it doesn't work at all if you can't see the sky. I've found that I use it when I drive now too. I'm chronically lost. It's a cool toy.

Will it do cadence and heart rate? My preference is to have everything in one unit, though gps is not in the budget for quite a while.

Both?

http://www.garmin.com/products/edge305/31526

I think what the OP is looking for is a bicycle computer that has a GPS built into it. Frankly I'm surprised Shimano has not developed a Flight Deck model of one yet. Give them time I'm sure they will.

I just bolt my eTrex to the top tube or handlebar. I couldn't care less about actually measuring cadence or heart rate at this point. The GPS tracks distance, time, average speed, top speed, current speed, and has a backtrack log and navigation ability to waypoints I've stored.


Fun stuff, but they're separate tools. A cyclometer can't replace a GPS if you need GPS features, nor vise versa.

GPS is accurate, at any given moment, to about 20 ft on average. That is, your reported position is anywhere within about 20 ft from the true position. This shifts back and forth as you move. In narrow streets with relatively little sky visible, you could easily appear to move completely opposite your true direction, because the error moves backwards faster than you move forwards! Not only that, but the error could shift between positions as far apart as 100-200 ft in a second...

GPS is fine for finding your way, but for precision work such as showing and recording speed and distance, it's not good at all. Unless you live in a flat desert with only a few very low buildings...

A cyclometer, when properly calibrated, is accurate in any condition, to about 0.3% or better. That is, the error in recorded distance for exactly 100 miles, for example, is only about 1500 ft or so at the most.

http://www.ja-gps.com.au/gpscomparison.html

Altitude is the complicated one, there are multiple ways each device calculates it.

Since a decent cyclocomputer only costs about $30-35 (i.e., Sigma BC1600 w/ cadence on eBay), why worry about it? Go buy one.:D

Both?

Agree. I had my Garmin Legend zip tied atop my Planet Bike cyclo-puter for yesterdays ride. Not for any particular reason other than I wanted to see if the two were in agreement on total mileage and average speed.

My plan is to drag the GPS along on the long meandering ride-abouts, where I find myself befuddled to where I actually am.

Unless they have a barometric altimeter, they're generally useless for elevation.

I've been using a GPS for years now. I have found they are very accurate most of the time and never need calibrating. I can switch the GPS to any bicycle I want without having to do a thing to it. Today's ride is an example. According to my GPS, I rode 23.36 miles. I then went to my Delorme mapping software, plotting out the exact route I rode today and used their software to determine the miles. 23.27 miles the software determined what I rode. That's pretty darn accurate.

Now in a downtown area with large skyscrapers or in a dense forest environment, the GPS probably isn't going to be as accurate as a probably calibrated cyclometer. But the key is PROPERLY calibrated cyclometer, meaning you take a ruler to measure your diameter of your tire rotation and use that figure, not the generic figure that the instructions tell you.

But there isn't a rule that says you can't use both? With a GPS you can download your rides to a map or use the GPS itself as a map. Cyclometers are very inexpensive and I used both on my tour in 2005.

I wouldn't say the altitude on a non-barometric GPS is useless but it isn't nearly as accurate as a probably calibrated barometric GPS altimeter. But if you got a low pressure system coming through your area and you don't calibrate the barometric altimeter, it isn't going to be accurate either. According to the manual, a barometric GPS altimeter should be calibrated every few hours.

Im going with both. Im keeping my current cyclometer and have ordered the Garmin Vista Cx.

Im going with both. Im keeping my current cyclometer and have ordered the Garmin Vista Cx.

Be prepared to shell out another $80 for decent maps.........*cough*bittorrent*cough*

I've been using a GPS for years now. I have found they are very accurate most of the time and never need calibrating. I can switch the GPS to any bicycle I want without having to do a thing to it. Today's ride is an example. According to my GPS, I rode 23.36 miles. I then went to my Delorme mapping software, plotting out the exact route I rode today and used their software to determine the miles. 23.27 miles the software determined what I rode. That's pretty darn accurate.

Now in a downtown area with large skyscrapers or in a dense forest environment, the GPS probably isn't going to be as accurate as a probably calibrated cyclometer. But the key is PROPERLY calibrated cyclometer, meaning you take a ruler to measure your diameter of your tire rotation and use that figure, not the generic figure that the instructions tell you.

But there isn't a rule that says you can't use both? With a GPS you can download your rides to a map or use the GPS itself as a map. Cyclometers are very inexpensive and I used both on my tour in 2005.


Sure, in a flat area, with few and low hills, and no forests or tall buildings, the GPS is fairly accurate. Your close match was likely luck more than actual precision, though errors could average out over longer distances to some extent.

The cyclometer, on the other hand, has almost no sources of error if calibrated properly (which is easy to do, if the unit allows detailed settings). Just ride a known distance (straight street/road measured on a detailed map, or in a GIS) and adjust the distance per revolution setting to produce that distance.

Both. I have CatEye CC-CD300-DWs on both the road bikes, and a CatEye Micro-Wireless on the MTB; I have a Garmin Quest which I've used on all the bikes for just over a year (i.e. all of 2006 season, and from about the fall of 2005).

The Quest is so convenient; I've ridden centuries, metrics, long and short rides, all without reference to a single map whilst on the bike - well, OK, planned routes have to be set up to start with, but how else would you do it? Even without pre-loaded routes, the maps in the Quest are quite adequate for navigation.

I've found the GPS to be such a boon for cycling - it certainly beats stopping at every junction to read a paper map! Of course it would be almost useless if the DoD decided to switch on Selective Availability, or if the maintenance of the constellation of satellites was less than stellar… (so to speak…) :rolleyes:

- Wil

Yes, GPS serves extremely well as a wayfinding tool. For recording distance travelled, average speeds and so on, it's not so good.

Be prepared to shell out another $80 for decent maps.........*cough*bittorrent*cough*


Thanks, *cough*Ill do that as soon as it arrives*cough* As a matter of fact Ill do that right now*cough* *cough*thanks again*cough*

Sure, in a flat area, with few and low hills, and no forests or tall buildings, the GPS is fairly accurate. Your close match was likely luck more than actual precision, though errors could average out over longer distances to some extent..

Not luck. I document and download every single ride I do. Very rarely is it off by even 1 percent. Now in all fairness, I do live in a rural, non-mountainous area but it is heavily forested. I guess if I rode around downtown NYC or along steep cliffs, it would be less accurate. The GPS is also great for average MPH, my unit always knows when I have stopped and will stop calculating a moving average speed and time moved.

But if I was really in need to be deadly accurate, I would get a cyclometer & calibrate it properly. But I find that being always under 1 percent is pretty darn good.

The only time I kick myself is the times I'll go walking for a geocache with my GPS while using it for bicycling. Or if I drive my car and forget to clear out the data. Then yes, it does mess up the data and I've got to reconstruct it from the GPS log taking out the distance and time walking or riding in the car.

Still a GPS is a great tool to have on a bike. And there's so many other things you can do with it besides bicycling.

I can't see them ever going back to Selective Availbity, too many public services and businesses now rely on GPS.

Yes, GPS serves extremely well as a wayfinding tool. For recording distance travelled, average speeds and so on, it's not so good.

Well, it might vary with different devices (although I don't see why it should), I've found that the Garmin Quest is pretty consistent with the cyclometer on each of my bikes, usually to within 1 or 2%.

- Wil

Thanks, *cough*Ill do that as soon as it arrives*cough* As a matter of fact Ill do that right now*cough* *cough*thanks again*cough*

That cough sounds bad. You should go see a doctor *cough* utorrent.

Nudge, nudge.

I did notice near the end of my tour that the cyclometer was showing a difference of 3 to 4 percent from the GPS unit while the beginning it was around 1 percent. I think it was the result of my tires wearing down and thus making the cyclometer more inaccurate. Air pressure probably also affects cyclometer accuracy, especially on mountain bike tires. The less pressure, the more tire that is constant with the road.

I probably should have done a wheel circumference measurement to bring the cyclometer back to being totally accurate but when you're riding 70 miles a day, it's not a real high priority.

Anyhow like I said with cyclometers being so cheap, no law says you can't have both a cyclometer and a GPS.

Pump your tyres up... :D

Im going with both. Im keeping my current cyclometer and have ordered the Garmin Vista Cx.

I've got both - an eTrex Vista Cx, and a Cateye Micro wireless. I pay more attention to the Garmin, since the numbers are bigger. I also use it to plot the elevation of the rides, too. The cyclocomputer I use essentially for overall speed and average speed.

Agree. I had my Garmin Legend zip tied atop my Planet Bike cyclo-puter for yesterdays ride. Not for any particular reason other than I wanted to see if the two were in agreement on total mileage and average speed.

My plan is to drag the GPS along on the long meandering ride-abouts, where I find myself befuddled to where I actually am.

Unless they have a barometric altimeter, they're generally useless for elevation.

the garmin vista has that you can get it at tiger direct for about 229.00 vista cx
gps is the only way to go
they are great

Im going with both. Im keeping my current cyclometer and have ordered the Garmin Vista Cx.
If the physical package fits your bars OK I'd bet you will be happy with that.
For training I use a Garmin Forerunner 301 and it is OK for that. For travelling
it can't compare to my old Garmin III+ with it's good antenna. Too bad the III+
won't fit on a truck!

Quick discussion of GPS accuracy: with 3 satellites strong and in view 2 dimension
mapping works and a 4th is required for 3D which provides altitude. I believe
that every vendors software will use more satellites but don't actually know that.
Imagine 3 satellites in a straight line at the zenith which are very close together.
Compare that with three satellites near the horizon as far as possible from one
another (120 degrees apart). The time signals and location from the first three
in a row will be very close together while the three in a triangle will be seperated
more. Any computational errors from the three in a line will be a much larger
proportion of their time differences compared to the big triangle satellite configuration.
Therefore the three in a row will have noticably more error.

Since the GPS system uses LEO (low earth orbit, not geosynchronous ~23,000 miles
away) satellites their congiguration in the sky above you changes all the time, as
does the accuracy. Both of my Garmins have a GPS screen which shows the actual
satellite configuration above you and something which indicates the error magnitude.

A rule of thumb that I've heard is that the altitude error is about 3 times the lat/long
error. I take that on faith but don't really know how it withstands scrutiny.

BTW, I've heard of people who are happy using their GPSs with Google Earth instead
of buying maps. (There may be a $20 US/year charge for the software that integrates
Google Earth with your tracks. I can't remember but I'll bet that Google can.)

My buddy hit a bump crossing a bridge and his GPS was knocked loose from the bracket and fell into the river never to be seen again. Pretty expensive gadget to be putting on a bike that could crash or otherwise lose it. Just food for thought. He sometimes loses signal and does not get accuracy that a computer would get.

Quick discussion of GPS accuracy: with 3 satellites strong and in view 2 dimension
mapping works and a 4th is required for 3D which provides altitude. I believe
that every vendors software will use more satellites but don't actually know that.
Imagine 3 satellites in a straight line at the zenith which are very close together.
Compare that with three satellites near the horizon as far as possible from one
another (120 degrees apart). The time signals and location from the first three
in a row will be very close together while the three in a triangle will be seperated
more. Any computational errors from the three in a line will be a much larger
proportion of their time differences compared to the big triangle satellite configuration.
Therefore the three in a row will have noticably more error.


Actually, four satellites is the minimum number of satellites required to give an unambiguous position. With three, there are two possible locations. Of course, if one of the two positions is outside the altitude limit of a commercial GPS unit, or below the geoid surface, then the other position can be assumed correct.

But, even in the best possible conditions, the average positional error is on the order of 15-20 ft.

My buddy hit a bump crossing a bridge and his GPS was knocked loose from the bracket and fell into the river never to be seen again. Pretty expensive gadget to be putting on a bike that could crash or otherwise lose it. Just food for thought. He sometimes loses signal and does not get accuracy that a computer would get.

You are riding a $1000+ machine that you could easily crash too. It's all about risks. My GPS has gone flying off the bike a couple of times. Each time was due to the fact that I did not latch it properly. Now I am more careful.

But I agree, if you just want something to keep track of miles etc. then a computer is the way to go. The batteries will last forever. I have a computer and a GPS. I only use the GPS when I want to go and explore a new area.

I am also thinking of buying a Garmin Edge. Not so much to use it to keep track of fitness but I write software for the Garmin that is used by other programmers. I would like to add the Edge fitness protocols to my software.

this is not an endorsement of a product, but all the 305 series Garmin EDGE are GPS-enabled cycling computers with a temperature compensated barometric altimeter.The lower priced EDGE 205 doesn't have a barometric altimeter and relies on GPS for the altitude reading.

If you get the Edge WITH cadence you also get a wireless speed/cadence sensor. you can also get it with heart monitor functions, or both heart rate and cadence.

it does not let you look at nice maps on the screen although it does do waypoint nav as well as offer post ride downloads that let you extrapolate your rides on maps via your computer.

it is primarily a tool for training and etc, designed first and foremost as a cycling computer.

Both?

http://www.garmin.com/products/edge305/31526Hello, I saw your pic of the GPS unit (looks good by the way) and was wondering if the mounting system you used could hold a small radio such as a Yaesu VX-6R. The radio is a small hand held (walkie talkie like) that I use during local emergencies. It is probably a little bigger than your GPS.

Anyway I'm looking for something to hold my radio while I ride.

Hello, I saw your pic of the GPS unit (looks good by the way) and was wondering if the mounting system you used could hold a small radio such as a Yaesu VX-6R. The radio is a small hand held (walkie talkie like) that I use during local emergencies. It is probably a little bigger than your GPS.

Anyway I'm looking for something to hold my radio while I ride.

Check out the Ram Mount. They have a wide variety of mounting systems.

Will it do cadence and heart rate? My preference is to have everything in one unit, though gps is not in the budget for quite a while.

If you are really looking for a do everything gps, look at the new Garmin Dakota 20.. Full screen gps with touchscreen technology.. Just a little smaller than the Oregon series but bigger than the 705.. The Dakota 20 allow you to use the Garmin HR and Cadence-Speed Functions.. Spoke to a salesperson at REI and these things are selling fast.. Seems like most places are out of stock right now.

they sell for 349.000 msrp but can be found online for 280.00.. Just a note that there is lower model Garmin Dakota 10, this model does not allow you to use your cycling specific accessories..


Garmin site: https://buy.garmin.com/shop/shop.do?pID=30926

REVIEWS
http://www.gpsfix.net/gamin-dakota-20-first-impressions/
http://gpstracklog.com/2009/10/garmin-dakota-20-review.html

Get the newer Garmin's with the H in the model (legend/vista), those have the more sensitive chipset and will work well between tall buildings or under bridges/tunnels/trees.
I use the neck lanyard wrapped around the handlebars as a safety, if mine would ever come unclipped it would just hang by the lanyard.
Another advantage of the GPS over a dedicated bicycle computer is taking it off the bicycle and on a walking/jogging/hiking trail. I also let mine log to a gpx file on the microsd card and then geotag pictures afterward.

The Dakota 20 allow you to use the Garmin HR and Cadence-Speed Functions
This is likely incorrect. The Dakota 20 is $350. The Edge 705 (which does do the HR/Cadence/Speed stuff using ANT+) is $500.

If you are really looking for a do everything gps, look at the new Garmin Dakota 20.. Full screen gps with touchscreen technology.. Just a little smaller than the Oregon series but bigger than the 705.. The Dakota 20 allow you to use the Garmin HR and Cadence-Speed Functions.... Just a note that there is lower model Garmin Dakota 10, this model does not allow you to use your cycling specific accessories.

I see in the Garmin Dakota owner's manual (page 29) (http://www.garmin.com/manuals/Dakota10_OwnersManual.pdf) that the Dakota 20 can be paired with an optional Garmin heart rate monitor and an optional Garmin bike cadence sensor, and display the values on the map page, the compass page, and the trip computer. What does a cyclist get from a fitness Garmin -- a Forerunner or Edge -- that is not available on the Dakota for less money than the Edge 705? I don't see any reference in the Dakota manual to power meters, or to the training functions, for instance.

I got a handheld Garmin eMap (now obsolete) eleven years ago in lieu of a cyclometer and would never go back (although I can see some sense in having both). Accuracy in general is very good and when comparing distances on club rides I find that my numbers are almost always somewhere in the middle of those recorded by folks with cyclometers. There are occasional glitches, especially in the urban canyons of city centers where the buildings hide much of the sky and also create reflected satellite signals, and also in dense redwood forests where the unit will lose satellite contact periodically. That can reduce the recorded mileage slightly, but it's very rare that the discrepancy is significant. OTOH, the max. speed reading can be thrown way off by a single false reading and I do sometimes get clearly spurious values for this.

But the benefits are numerous - being able to see at all times where you are on the map, what alternate routes will get you back if you're getting tired or running out of time, keeping records of where you went, how fast you were, etc. I also upload many of my rides and associated pictures to the everytrail.com site, such as this one from yesterday:
http://www.everytrail.com/view_trip.php?trip_id=429752
[the site looks at the time stamps on the GPS tracklog and compares to the times embedded in the picture files to determine where each photo was taken]

The GPS is especially handy while touring since the maps contain the locations of millions of businesses. Being able to find the location of the next grocery store, bike shop, library, or emergency motel has come in very handy at various times.

I save all my gps logs too, uploaded to everytrail
http://go.bikeforums.net/?id=42X129555&url=http%3A%2F%2Fwww.everytrail.com%2Fmy_trips.php%3Fuser_id%3D76636%26order%3Dtrip_date

This is likely incorrect. The Dakota 20 is $350. The Edge 705 (which does do the HR/Cadence/Speed stuff using ANT+) is $500.

This feature is available for the Dakota 20, the Oregon 400 series and 550 series.. I think the size of the oregon is why many people did not use it for cycling, but this new model is a little more compact in size, similar to the etrex hcx series in size but more useable screen size.. The screen is being touted in reviews as working better in direct sunlight versus the oregon series..I have seen a couple of reviews on MTB sites praising the new Dakota model..

You are right, why pay 500.00 for a 705 when you can get the dakota for less, even if you factor in buying optional maps, it is still less in price..

Dakota screen size: 1.43"W x 2.15"H
Edge 705 Screen Size: 1.37" x 1.71"
Oregon Series gps: 1.53"W x 2.55"H

I see in the Garmin Dakota owner's manual (page 29) (http://www.garmin.com/manuals/Dakota10_OwnersManual.pdf) that the Dakota 20 can be paired with an optional Garmin heart rate monitor and an optional Garmin bike cadence sensor, and display the values on the map page, the compass page, and the trip computer. What does a cyclist get from a fitness Garmin -- a Forerunner or Edge -- that is not available on the Dakota for less money than the Edge 705? I don't see any reference in the Dakota manual to power meters, or to the training functions, for instance.

Interesting.

I'd think that the device displaying the ANT+ data would have to know about what kind of data is being sent to it. For example, since there is no way of telling the speed transmitter what the wheel size is, the conversion of wheel rotation pulses has to be converted to speed at the display/head unit. To be able to display power, I'd guess the display/head unit would have to be preprogrammed to display power. This wouldn't be hard to do but it might be the thing, along with the training functions, that distinguishes the Dakota 20 from the more-expensive Edge 705.

The other thing that distinguishes the Edge 705 is a handle bar mount!


This feature is available for the Dakota 20, the Oregon 400 series and 550 series.. I think the size of the oregon is why many people did not use it for cycling, but this new model is a little more compact in size, similar to the etrex hcx series in size but more useable screen size.. The screen is being touted in reviews as working better in direct sunlight versus the oregon series..I have seen a couple of reviews on MTB sites praising the new Dakota model..

You are right, why pay 500.00 for a 705 when you can get the dakota for less, even if you factor in buying optional maps, it is still less in price..

Dakota screen size: 1.43"W x 2.15"H
Edge 705 Screen Size: 1.37" x 1.71"
Oregon Series gps: 1.53"W x 2.55"H

It's interesting that Garmin doesn't make this feature very apparent!

The $500 Edge 705 includes the $60 HR monitor and the $13 bike mount. The speed/cadence sensor is an extra $60.

It looks like the Dakota 20 can display non-computed data (basicallly, labeled counts/second), like HR and cadence.

(If I go with at GPS unit, I'd want one that would work for the car, hiking, kayaking, and bicycling. I wonder how well-suited for use in the car the Dakota would be.)

I use the velocomputer on my BlackBerry Storm 2. It doesn't have an immediate display of the path you are on, but it does output quite nicely into google maps and earth. It'll record distance, speed, acceleration, cadence, and position. More than decent for a 15 dollar app.

I work for them but I also use it so, I hope this isn't solicitation. Lemme know if it is and I'll delete the post.

Rob

I've got a Garmin Forerunner 205, and have logged over 10,000 miles on it. I absolutely love it. But I'll soon be wearing a Forerunner 405cx, which will allow use of HRM and cadence sensor. For me, the key feature of a GPS is that in addition to being able see my speed and distance on my handlebars, when I get home I sync it with the computer and see a map, my speed at any given point, etc. and so forth. One big disadvantage of a GPS is that while they are a lot more accurate overall than one person in this thread seems to think, the speed displayed at any given moment is guaranteed to be a few seconds out of date. So if you're leading a paceline and need to maintain a dead level speed, a bike computer is essential. (I believe that the full cadence setup measures speed directly and shows that on the LCD, but I'm not 100% certain.)

I've got a Garmin Forerunner 205, and have logged over 10,000 miles on it. I absolutely love it. But I'll soon be wearing a Forerunner 405cx, which will allow use of HRM and cadence sensor. For me, the key feature of a GPS is that in addition to being able see my speed and distance on my handlebars, when I get home I sync it with the computer and see a map, my speed at any given point, etc. and so forth. One big disadvantage of a GPS is that while they are a lot more accurate overall than one person in this thread seems to think, the speed displayed at any given moment is guaranteed to be a few seconds out of date. So if you're leading a paceline and need to maintain a dead level speed, a bike computer is essential. (I believe that the full cadence setup measures speed directly and shows that on the LCD, but I'm not 100% certain.)

My edge 705 is setup that way, and shows near-instant changes in speed. Cadence and elevation is always a few seconds behind.

(I believe that the full cadence setup measures speed directly and shows that on the LCD, but I'm not 100% certain.)
That is correct. The Garmin speed/cadence detector uses a magnet on the wheel and another on the crank. It is as immediate as any other cyclometer. GPS-measured speed is quite accurate but it takes a while to get enough detail and the satellites have to be visible. I'd guess that GPS speed is more accurate at higher speeds (and longer distances). The GPS is probably not so good at measuring your speed doing slow donuts in the parking lot!


So if you're leading a paceline and need to maintain a dead level speed, a bike computer is essential.
Good point. There has to be some reason for the added expense of the seemingly "reduntant" speed measurement!

The Dakota uses the same handlebar mount as the Oregon/Colorado series.. Readily available at 10.00..

https://buy.garmin.com/shop/shop.do?pID=11411

Keep in mind that a GPS works by using satellites. I love mine, but every once in a while, it'll blip and say my max speed was 400 MPH or something like that. In general though, it works well. It doesn't work as well if you use it amongst tall buildings and it doesn't work at all if you can't see the sky. I've found that I use it when I drive now too. I'm chronically lost. It's a cool toy.

A lot of cyclometers will do that as well.

A handheld GPS is NOT a good replacement for a cyclometer. It is not designed for vehicular use. Further, the altitude readings from a garden variety GPS wildly inaccurate. I guess someone reasoned knowing WHERE something is AND it's altitude are pretty critical to putting a missle on in it ;-)

In any case, the Garmin bike GPS units have barometric altimeters in addition to the GPS reading. You can also use a wireless speed sensor that will work regardless of cloud/leaf/building cover. You can also use wireless cadence and heart sensors with them. These are also things cyclists consider important.

A lot of cyclometers will do that as well.

[QUOTE=BearSquirrel;10106943]A handheld GPS is NOT a good replacement for a cyclometer. It is not designed for vehicular use.
Note that a "handheld" GPS doesn't work any differently than the ones desgined for "vehicle" use (ie, ones designed for navigation use in cars. The problem with any of them is that they don't work very well at registering speeds at low speeds (this ends up not really being a problem for cars, because they are relatively fast).


Further, the altitude readings from a garden variety GPS wildly inaccurate.
Anyway, the real problem with GPS-only altitude measurements is that the GPS system was designed to locate position on 2-dimensional surface because that was really the only goal. (Adding precise altitude measurements would have made the system much more expensive.)


I guess someone reasoned knowing WHERE something is AND it's altitude are pretty critical to putting a missle on in it ;-)
Actually, knowing the altitude is irrelevent for targetting missiles since it's generally good enough to be able to hit the ground that the target sits on!


In any case, the Garmin bike GPS units have barometric altimeters in addition to the GPS reading.
To be able to determine altitude with more accuracy (to be any sort of use to people), the device needs more information than satellites can provide. GPS units use barometric pressure to provide that extra information.


You can also use a wireless speed sensor that will work regardless of cloud/leaf/building cover. You can also use wireless cadence and heart sensors with them. These are also things cyclists consider important.
The Garmin Dakota 20 (and some of the Oregon units) will display the data that cadence and HR monitors provide because the transmitters use ANT+ and these Garmin units are ANT+ recievers.

The Garmin Edge 705's one special feature is that it will display speed from a wheel-based transmittier by allowing the wheel size to be entered into the display unit. The other advantage of determining speed this way is that it doesn't need access to satellites and it is more accurate at low speeds.

Note that a "handheld" GPS doesn't work any differently than the ones desgined for "vehicle" use (ie, ones designed for navigation use in cars.

No argument. But there is a big difference in how the different products are used. A handheld unit for field use has no navigation, so you wouldn't want to use it in your car. Likewise, a handheld GPS isn't optimized for bicycle use.




The problem with any of them is that they don't work very well at registering speeds at low speeds (this ends up not really being a problem for cars, because they are relatively fast).

Agreed, they don't work well at low speed. But speed has nothing to do with it. The issue is that GPS measures speed by measuring the difference in position. It does so at specific time intervals. So if you move in anything other than a straight line, the GPS will under-report your speed and distance travelled.



Anyway, the real problem with GPS-only altitude measurements is that the GPS system was designed to locate position on 2-dimensional surface because that was really the only goal. (Adding precise altitude measurements would have made the system much more expensive.)

No you're dealing with a 3-dimensional structure. Even if you use spherical coordinates, it's still a 3-dimensional system. I don't think the military would have forgotten that they have aircraft when they designed the system.



Actually, knowing the altitude is irrelevent for targetting missiles since it's generally good enough to be able to hit the ground that the target sits on! The ground sits at a specific altitude. I'm pretty sure if the missle is flying too high, it will miss the target. It's a 3 dimensional system and the only reason it would not matter is if you dropped it from a standstill immediately above the target.




The Garmin Edge 705's one special feature is that it will display speed from a wheel-based transmittier by allowing the wheel size to be entered into the display unit. The other advantage of determining speed this way is that it doesn't need access to satellites and it is more accurate at low speeds.

Yes, cyclocomputers make better cyclocomputers.

No argument. But there is a big difference in how the different products are used. A handheld unit for field use has no navigation, so you wouldn't want to use it in your car. Likewise, a handheld GPS isn't optimized for bicycle use.
It appears that the Dakota provide for turn-by-turn navigation. There's no real technical reason that a handheld can't have turn-by-turn navigation.


Agreed, they don't work well at low speed. But speed has nothing to do with it. The issue is that GPS measures speed by measuring the difference in position. It does so at specific time intervals. So if you move in anything other than a straight line, the GPS will under-report your speed and distance travelled.
Speed is part of it because slow speeds (moving in a straight line) means your position doesn't change very much. And there is also margin of error with measuring position too. That means one would probaly get unreliable speed measurements going around in tight circles even if your speed is high.


No you're dealing with a 3-dimensional structure. Even if you use spherical coordinates, it's still a 3-dimensional system. I don't think the military would have forgotten that they have aircraft when they designed the system.
No, GPS works worse for altitude and it's an inherent issue (maybe, related to the number of satellites). The military isn't forgetting: There are other ways of determining altitude (more accurately).

Of course, the cyclist is also going to be more sensitive to errors in altitude than he would be with errors in surface distance (a 500 ft climb is "significant" while a 500 surface distance traveled isn't).


http://www.gpsinformation.net/main/altitude.htm


Altitude error is always considerably worse than the horizontal (position error). Much of this is a matter of geometry. .... As a result, of this geometry the calculated solution for altitude is not as accurate as it is for horizontal position. Almost any calibrated altimeter will be more stable at reading altitude than a GPS. ... First, the geodetic model of the earth can have much more than this amount of error at any specific point and Second, you have the GPS error itself to add in. As a result of this combined error, I am not surprised to be at the seashore and see -40 meter errors in some spots.
...
In any case, it is extremely unwise to overly depend on the altitude readout of a GPS. Those who use GPS altitude to aid in landing their small plane should have their insurance policies paid up at all times.




The ground sits at a specific altitude. I'm pretty sure if the missle is flying too high, it will miss the target. It's a 3 dimensional system and the only reason it would not matter is if you dropped it from a standstill immediately above the target.
I'd guess cruise-missles have some sort of radar to keep them off the ground when flying. And, if you are at the target, you can have the missle steer into the ground. That is, nothing needs to be missed.