[Qgis-user] Finding a flat spot

[Kirk Schmidt]

Hi Morgan:

This is an interesting question.  I am assuming the gpx tracks do not 
have elevation data, and if so, they may not have the accuracy needed 
for the 12 inch delta in elevation.  Also, the device you used to record 
the daa may be optimized to minimize file size, therefore is is unlikely 
you have usable elevation data in the gpx files.

You need to generate a raster digital elevation model (dem) or access 
one for your target area.  In contrast to vector data,  a raster is 
typically a continuous grid and for elevation data, each grid would 
contain the average elevation for that spatial location.  So if each 
cell in a dem is 10m x 10 m (30.49' x 30.49'), it would not be able to 
find a flat area that is 100 ft2 (10' x 10'). This is referred to as the 
resolution of the raster.

If you can locate aerial lidar data for your area, the associated dem 
typically has a high resolution and would meet your needs. You could use 
the contour lines from a georeference map, however, you would have to 
digitize the lines and you would be limited by the contour interval.  If 
it was 6", then you could find what you are looking for, if it is 5, 10 
or 30 feet, there would be too much approximation to reliable find flat 
areas.  The workflow would be to digitize or source the contour lines, 
convert them to points at high density> use these to generate a raster 
at a high enough resolution to find low slope contiguous cells that are 
less than 100 ft2, so perhaps 2.5 to 3'.

If you can source a high resolution dem, then you can generate a slope 
raster from the dem and us that to find contiguous areas of 100 ' or 
more that have a slope of less than 10 % (1 foot over 10 feet).

This is a conceptual answer to your question.  If you proceed, and can 
source a high res dem, than re-ask the question and the group can 
provide the specific steps to reach your gola.

Kirk Schmidt


On 10/21/2025 9:27 PM, Morgan Fletcher via QGIS-User wrote:
> I use qgis mostly for visualizing gpx tracks, georeferencing old maps, 
> drawing vectors to reveal or highlight information related to a map or 
> physical location.
>
> I have a real-world need, a wish list item, and I wonder if QGIS might 
> be able to solve it.
>
> I want to know where the flat spots are. Given a bounded area on a 
> map, could I ask QGIS to find me all locations that are at least 100 
> square feet, with less then 1' vertical delta across that space? Yes, 
> I'm trying to find flat camping spots.
>
> Morgan
>
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-- 
Kirk Schmidt, MScF, BScF, RPF
General Manager
Nortek Resource Solutions Inc.
26 Church Road
Sutherlands River, NS
B0K 1W0
Tel (902) 922.3607
Email: kirk at nortekresources.com
Web: www.nortekresources.com