[OSM-talk] Contour lines

[Andrew Errington]

On Tue, 13 Apr 2010 17:59:47 Steve Hill wrote:
> On Tue, 13 Apr 2010, Gregory wrote:
> > What makes you think the Yahoo! imagery and/or the contour lines shown on
> > the Cycle Map(these  contours are from SRTM data, not OSM) are more up to
> > date or more accurate/precise than the existing water body drawn in OSM?
>
> Indeed.  Also, ISTR the cycle map fakes up realistic looking contours for
> void-filling doesn't it?
>
> I don't think the SRTM contours are accurate enough to use for tracing
> bodies of water - you need to walk the perimeter with a GPS or use aeriel
> photos.  SRTM data _may_ be useful for guestimating flowing water courses
> that can't be otherwise surveyed, but for this I think you'd really want
> to be computing stuff off the raw DEM rather than manually adding
> features using the (already quite heavilly processed) contour lines.

My goodness!

Care to estimate the accuracy of the Yahoo! aerials?  In this region they are 
a bit fuzzy, and not very well aligned with WGS84.  My GPS accuracy (reported 
by the device itself) is at best 4m, but in mountainous regions or in cities 
it is 5m or 6m at best, often worse.  How accurate is my mouse when I click 
on a pixel?  Visually I am interpolating the (inaccurate) GPS trace to get a 
smooth, pleasing curve or shape.  Although we strive for accuracy you have to 
remember we are not surveying, we are making a map; there is a difference.

In the case of the reservoir, the contour lines line up remarkably well with 
other map features (which were recorded independently of the contours, but 
plotted together to make the OSM cycle map).  Rivers follow gullys, roads 
snake appropriately over dense contour lines, etc., so I see no reason not to 
use them.  Furthermore, the reservoir level changes throughout the year, 
depending on the weather.  Yahoo! may have snapped a picture of the average 
level, or a minimum.  If I go there with my GPS I cannot get to the water's 
edge- it's too dangerous, so I have to estimate where the waterline would be 
based on my (inaccurate) GPS and (inaccurate) memory and notes.

The previous version of the reservoir is jagged and rough- a few points 
clicked around the perimeter.  The body of water is about 16km long.  I wish 
to make the reservoir outline better.  I cannot get an aerial of the 
reservoir when full.  I cannot walk around the water's edge.  I don't have an 
RC model speedboat with my GPS on it (yes, I thought about it) so I will use 
the next best thing at my disposal.  SRTM contours.  The next guy that comes 
along will improve it, as I have done, and as we all do.

Having said that, I have just compared my changes with Google aerials, Google 
Maps, and another two mapping providers.  They are all different, but mine is 
a little high, so I am going to choose the next lowest contour.

Best wishes,

Andrew