GPS precision

bentes

Hi guys!

I don't know if I should post it here or on the where to buy section, please escuse me if I'm wrong.

So, my question, do you know what's the precision of a usual MTB GPS?

I've been eyeing the gps's for quite a while but never had the perfect excuse to buy one.
But now a situation came up where I have to "cartograph" a series of family lands, and thought that this would be a great excuse to buy one!

I will need to pinpoint as precisely as possible a few marks that delimit the lands, and If possible transpose these coordinates to a map. (google maps or something).

So, do you have any idea of the precision of the Edge 605, 705, or dakota? And are they any good for this task? Do you have any recomendation?

Thank you very much in advance for any help.

RealMan

My edge 305 I think has a minimum accuracy of about 4m? And I imagine its similar in the other Garmins.

Maybe you want a surveyor though?

bentes

Thanks RealMan.

I've googled it and don't seem to find this exact information anywhere.

Thanks for the quick response. Was hoping these new systems had better precision though...

unixnerd

I write software for offshore positioning.

For high accuracy you want differential GPS (DGPS). It's possible to do it on the cheap, a mate of mine once had his own DGPS base station!

cee

RealMan wrote:

My edge 305 I think has a minimum accuracy of about 4m? And I imagine its similar in the other Garmins.

Maybe you want a surveyor though?

thats not a claim that garmin actually make though as a number of factors influence GPS accurracy...

simply put...the more satellites your receiver can 'see' then the more accurate it is likely to be.

from a quick google...most specs indicate an accurany over a percentage of time...so perhaps garmin say within 4 metres 95% of the time. That other 5 percent (1 in 20...so not insubstantial) they simply do not say...and they already caveat it with an assuming satellites can be seen disclaimer.

its not going to give you a perfectly accurate race timing system (i.e. where the requirement for accuracy to the metre within a tenth of a second can mean the difference between 1st and 10th place.) however is it most likely accurate enough for the majority of 'seeing where you are' uses.

this is a good article http://www.gartrip.de/long.htm which records the position indicated on a gps which was in a fixed position. you can see this yourself if you leave your unit in one place...the little arrow seems to move around a bit.

NDawn

I wonder if the Russians will make their GPS publically/worldwide available once they reach global coverage?

But then again, I assume different GPS satellites require different receivers in the hand held unit?

bike-a-swan

I'm sure I read somewhere they were looking at integrating it to some degree. I imagine either they'll broadcast a matching signal to the US ones, or we'll be seeing new receivers that can accept either.

unixnerd

The Russian receivers are likely to remain much larger and heavier than GPS, probably no use for cycling.

stumpyjon

My Extrex Vista can usually tell which side of the road I'm on, particularly clear on an out and back ride.

CFS

I would take the accuracy stated by your receiver with a pinch of salt. It may say 4m but is that 95 percentile, 50 percentile or 1 sd, etc? The manuals often don't tell you.

With decent satellite coverage you will probably get sub 15m and with WAAS enabled you may even get sub 3m.

I know my old E-trex has lied a bit and my Nuvi sometimes get fooled by slip roads parallel to a motorway maybe 20m away.

You could leave it in one place for a while and just record the position and average it out. You could also place it on a surveyed spot eg) Trig point to see what that error is but that is probably all a bit tricky.

You also need to ensure that the GPS datum (eg WGS 84) and co-ordinate set matches any map you are overlaying the data on.

x-isle

It really depends on the GPS unit and of course where you are.

If you spend a lot of time in thick forest then you need a unit that can cope.

I use the awesome SatMap Active 10 and it is pretty faultless. It updates every second which is quick enough for Biking use. It also uses real OS Maps and you can decide on 1:50K or 1:25K.

The only downside is the cost of the maps, the 1:25K (some people say that 1:50K is good enough, but I prefer 1:25K) can be pretty pricey.

Here is an example of the output run through GPS visualizer, this is of the new Monkey Trail at Cannock Chase and it shows altitude.

http://www.craigrogers.co.uk/gps/chase- ... vation.htm

It's pretty acurate as the lines are bang on.

paulbox

Depending on what kind of phone you have (if it doesn't have built in GPS you might be able to connect to a blue tooth GPS unit which are pretty cheap), look for an application like:

http://www.endomondo.com

I use it to track my rides, maps automatically when you upload to their website as well as tracking changes in altitude.

I started a thread on it in this section last week, other people have suggested other alternative applications on there.

bentes

Thank you for all your input guys!

x-isle, that mapping seems pretty detailed to me, I don't know if I'll need to be any more detailed than that.

Do you guys think the SatMap Active 10 is more accurate than the Garmin ones?

Thank you!

x-isle

Eeek, I don't want to start that debate off......

I've never used a Garmin so can't really say in my own words which one is better, however, all I can say is the SatMap has been the choice of a lot of reviewers if you take note of them. The Gadget show loved it.

It certainly does everything I want from a full GPS unit, the screen size is great, it doesn't have touch screen which for this purpose I think is a bonus, it's very robust, It had a full on hit with a rock a few weeks back when I lost it on a some rocks resulting in just a few scratches on the screen cover (which is replaceable for £15). It didn't even come off the bar mount. The POIs is easy to record and it also does Geocaching.

It updates every second which means it can also be used as a speedo/computer type thing. I have the 4 data bar enabled which gives me Speed, distance travelled, overall ascending and current time, but these can be tailored to whatever you want to display.

timmys

I'd say my Garmin 205 is generally accurate within 2-3 metres. Here's an example which might give you some idea of it's variance - four separate tracks recorded weeks apart (also Cannock BTW );

cavegiant

The best I have found is the newer HTC phones.

They can download satalite positions from the net, use normal GPS, trig of phone masts (normally terrible) and use DGPS.

DGPS is the one you need for accuracy.
I can remember the satalite number...15 I think?!

It is not a satalite but a ground station that you can reference off.

lots of books on this subject, some of them interesting.

You must also take into account maps are not accurate for small details, big things like hills, spot on, but if you think that surveyor tracked that stream exactly prepare to be surprised.

paulrockliffe

How are you running DGPS on a HTC? Are you sure you don't mean AGPS? AGPS isn't any more accurate, it just gets a lock faster and can give you a rough (within a few km) position when you haven't got a lock.

If you really want to survey with any more accuracy than just drawing lines on the map based on your own observations, you could hire a Trimble. These use proper DGPS and can be accurate to within a few cms.

abductee

http://www.kowoma.de/en/gps/accuracy.htm

I think you need EGNOS for best accuracy. Also some Garmin models do waypoint averaging which should improve the accuracy of the point that you save by taking the average of a number of measurements.

stubs

I read an interesting article on GPS accuracy in a mountaineering forum. The conclusion of the article "The World is littered with the corpses of stupid mountaineers who thought there Gps was accurate"

Happy Harry

Like any man I can understand the need to regularly buy new toys and gadgets and I'd love to need a GPS. But after so many years of using OS Explorers and a compass I'm struggling to understand what a GPS offers over the tradional £8 sheet of paper system.

If I can't justify it to myself, what hope have I got of convincing the memsahib :roll:

Perhaps I'm just getting old ...

Buckled_Rims

stubs wrote:

I read an interesting article on GPS accuracy in a mountaineering forum. The conclusion of the article "The World is littered with the corpses of stupid mountaineers who thought there Gps was accurate"

As a cyclist and mountaineer (but not a good one) I'd rather trust a GPS unit then a compass near magnetic rock.

Of the last 8 years I've used a GPS on hundreds of hills, only once or twice has the actual position been more then 10 metres away when I overlay it on a 25k Memory map. The usual accuracy for walking is 2 metres, on the bike it might be 3 metres because of the increase in speed as the unit takes a measure every second or so (can't remember my settings).

I love overlaying routes on Google Earth. Sad ain't it?

stubs

Buckled_Rims wrote:

stubs wrote:

I read an interesting article on GPS accuracy in a mountaineering forum. The conclusion of the article "The World is littered with the corpses of stupid mountaineers who thought there Gps was accurate"

As a cyclist and mountaineer (but not a good one) I'd rather trust a GPS unit then a compass near magnetic rock.

Of the last 8 years I've used a GPS on hundreds of hills, only once or twice has the actual position been more then 10 metres away when I overlay it on a 25k Memory map. The usual accuracy for walking is 2 metres, on the bike it might be 3 metres because of the increase in speed as the unit takes a measure every second or so (can't remember my settings).

I love overlaying routes on Google Earth. Sad ain't it?

The problem is people start to put 100% faith in there gps. If the arrow points forward they go forward and with depressing regularity straight into trouble because they have stopped using there eyes. Witness the motorists who get stuck in narrow dead ends because they blindly followed the gps without looking out of the windscreen.

Gps is a great tool but if you drop it, the batteries go dead or you just plain downloaded the wrong route your stuffed. There are too many people out there now who plain dont know how to navigate without the little arrow to follow.

ps I have just bought a Garmin Dakota 20 and I am loving it

Buckled_Rims

stubs wrote:

The problem is people start to put 100% faith in there gps. If the arrow points forward they go forward and with depressing regularity straight into trouble because they have stopped using there eyes. Witness the motorists who get stuck in narrow dead ends because they blindly followed the gps without looking out of the windscreen.

Actually I forgot about my car sat nav. The first time I used it, it told me to drive through a brick wall :!: The road was blocked up 20 years ago.

mbukfan

just a little note that WAAS is a US thing, EGNOS will do the same thing in the UK and Europe.
That claims <3m

Been almost 2years since I reasearched it but I think unless you spend more monely on very specialist kit.