[OSM-talk] augmenting contour data with gps track logs

[John F. Eldredge]

I have heard that the altitude data available to civilian GPS units (as opposed to the US military units) has deliberate errors built in, in order to make it harder for someone planning for a future artillery attack.  There is said to be a separate, more accurate, but encrypted, altitude signal for US military use.  Remember, the GPS satellite constellation was put up by the US military.

-- 
John F. Eldredge -- john at jfeldredge.com
"Reserve your right to think, for even to think wrongly is better than not to think at all." -- Hypatia of Alexandria

-----Original Message-----
From: Frederik Ramm <frederik at remote.org>
Date: Tue, 29 Dec 2009 00:20:09 
To: Robin Paulson<robin.paulson at gmail.com>
Cc: OSM Talk<talk at openstreetmap.org>
Subject: Re: [OSM-talk] augmenting contour data with gps track logs

Hi,

Robin Paulson wrote:
> ok, i didn't realise that. what levels of bad precision are we talking
> about; i assume it varies form model to model; how does it compare to
> the precision of srtm?

I'm not really an expert in this but Internet sources say that altitude 
error is *at least* factor 1.5 compared to x/y error. My personal 
experience (with a decent & recent Garmin device) is that a 200ft 
altitude error is not unusual.

SRTM3 reportedly has a maximum error of about 20ft (but I haven't 
verified that myself).

> i would envisage it being used in favour of the srtm data (which is
> very coarse, as you point out, hence my interest in replacing it)

There are also patches of better altitude data available. What we 
normally use is SRTM-3, but for the area of the USA there is also SRTM-1 
with triple the resolution.

> my idea was to extract a series pf lat, lon, alt triplet from the gps
> tracks, and use this to build a 'mesh' of the earth's surface, which
> would be stored independently of the points/ways which we create at
> the moment. the implicit assumption being that wherever the post box
> or any other object is placed on the globe, it takes it's height from
> the underlying mesh. from this mesh, we can then generate contours, 3d
> models, etc, etc as we do at present with the srtm data

That would certainly be a good start. I do however think that special 
editors and the ability to manually edit that mesh will be required 
sooner or later. You might, for example, have very a few very precise 
measurements that you want reflected in the data, or you might want to 
correct for an erroneous track or something.

The folks from OpenSeaMap are, by the way, thinking along the same lines 
for bathymetry (ocean depth) data. They might get access to a very 
coarse international data set which they can use as a basis, but then 
want to refine that using spot measurements or other sources. It appears 
that well-equipped ships, when en-route, automatically broadcast ocean 
depth measurements of some kind which can be used to improve data.

>> In my eyes, a separate project "OpenTerrainModel" or something like that is
>> called for, the results of which could seamlessly replace our usage of SRTM
>> data today.
> 
> exactly
> is there an api for accessing the gpx files en masse? or a gpx.planet download?

Lars Francke is working on something like it at the moment, see "Export 
of GPX data" over on the dev list.

Bye
Frederik

-- 
Frederik Ramm  ##  eMail frederik at remote.org  ##  N49°00'09" E008°23'33"

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