Topographical data

Does anyone know if the topographical data stored by the gps’s is going to be used for map making. I read somewhere that some data is/(is going to be) provided by free satalite data, but in cities the density of roads is such that extreamly acurate land shapes can be made out.

The accuracy on elevation is not always so good, You can still collection data and not be aware of the problem.
It could bee a lot of bad data in the database.

SRTM elevation data could be imported into the database.

Nick import STRM and uses it in his map, on the dev server, It will perhaps be used sometime on OSM.

Is that from freemap, as it looks very like it. It is very good, but mabye having contours is a bit complicated, is there any plans to switch certain layers off? I hope to see it used as the main view soon.

Per: When i look at the topographical data off my gps it seems to be quite consitent, but at the start of trips is often well of, and instead of a quick correction, it gradually comes back toward the correct measurments. Wonder if this is just becuase of averaging, and the later readings being more acurate as it picks up satalites.

Is the dev server, just a testing spot before it goes onto the osm site?..Quite a hypnotic page with the satalite image still behind the array of brightly coloured lines.


When i first learn about GPS, about 20 yers ago, they talk about the problem with elevation. Maybee it have with that you need a satelite in ‘zenit’ to get good data for calc.
If I look at the NMEA output from my GPS I can se that elevation have less accuracy, but like you see Ben. with averaging it be werry good, but not in the begining.
I always have an eye to the elevation figure, if they dispers I can suspect any problem with all of my data.

(I sould ask my freend ho is a specialist, if he can explain to me more precisly)

I imagin that anyone that has searched the internet about GPS topics enough to find themselves here may have used this site; but it converts gps data into a visual altitude map, represented by colour. Not much that i could think of doing with the outcome other than admire it though, but it was satisfying to colour areas in.

just want to add, that the import of evelation into OSM is not recommed, because we realized, that the heights are very inaccurate.

Try it out: measure your height on subsequent dates on the same location. Measure the height using differen gps units. It differs greatly! :frowning:

Ciao, Imi.

Yes one would need acouple of underground satellites to get a good accuracy for relevation… :wink: Though the relativeness of elevation works pretty good for me, i.e when I go over a hill that is ~20m it gets noticed by the GPS receiver pretty consitent.

Yes, on a boat in the sea, the GPS reads 15 metres.

I actually find mine very reliable. When i set off it can be out for 5 miniuts or so, but after that the data seems consistent. When i was at the sea last time i walked along the low tide and it said -15 feet… it was an extremly low tide, although it was still above the ‘0’ mark. So not shore where 0 was set at on my gps.

When i measured a point in norfolk a while ago, i got 202ft, The OS map said i was at 62 meters (“hill” top) (202.74ft) and it was 206.7ft according to Although I don’t no how acurate it is. The persons house who i was said it was 205 feet (not shore his source)… But those figures are all withing headheight. I compaired quite a few other points to maps and that website, and the norfolk example was no exception.

I don’t really understand why it would be less reliable than a x,y coordinate, since isnt it the same calculations that would work out ‘z’? (just requiring an additional satalite)

Must be a much more accurate GPS than mine. My travel one was the one that comes with the Tom Tom navigation software, so it is probably not that accurate. There is also one in our boat, which is quite old now.
As I said before, SRTM data is probably the most accurate most-of-earth coverage we have just now.
GPS elevation data is only for the track that was logged, and these don’t cover a very large area of the earth (yet).

Let me first state that elevation is magical (relative), the GPS logs doesn’t spit out the same elevation that you get on maps. It might, if there is software doing stuff with before it’s spit out to the user.

Elevation is a bit complicated, bascially it’s because you have all the satellites above you (very high above you), So you have less information about how far away you are in height. But I don’t know enough to explain this, it’s just a fact that I can repeat. :wink:

The main issue is that the GPS satellites are 20,200 km above the surface. A few metres are pretty small compared to this, so it is very difficult to get height on a normal GPS.
DGPS might be better.

hmm…heres a post to expose my ignorance. Don’t your X Y coordinates also come from 20,000 km up? I asumed the distances are calculated by imbedding the time in the packet that leave the satalight. Then your gps, knowing where satalites are placed, calcualtes its position…

anyway… if the height of the satalite is also imbedded, why doesnt the exact same calculation just work out the land hieght. Shorely 3 satalights would even be enough to tell you that you are either 100ft above sea level (for example)…or 40,000 km above the sea, with there being no points for guessing the more likely figure.

Also if a satalite is x distance up, then that is an inportant figure becuase the hypotinuse, to you, is longer than just the satalites vertical hiehgt…hmm the only way i can think of this making sence is if your gps device knows where the satalites are relative to one another prior to recieveing a signal.

Therefore I still cant see why the altitude is less acurate than xy coordinates still.

The signals travel at 300 million metres per second, so it would be difficult to distiquish a few metres height wise. See for more information on how GPS works.

You still need a satelite above your head before you can mesure a good distans. Most of the time the satelit is on the horisont. In many directions!

I have good data after a time. from sea level to 1500m (1474 for more exactly :slight_smile: )

Bruce89: I get that the speed of light is kinda fast…!, but the speed of light doesnt change when it comes to working out x,y co-ordinates. So I’m still puzzled why hieght is less acurate than x,y co-ordinates. I have read that site before but i cant find the answer im looking for

The only thing that i can think of that makes unacturate measuements is the speed of the clock in your gps device. With the gps device having a less speedy clock in it, it can only meausre ‘relitvely’ acuratly, so taking the center point between many semi acurate points would result in a more acurate point…(averageing in effect) but this still doenst really make me anymore the wizer becuase the altitude could be calulated by this same means shorely.