[Qgis-user] Finding a flat spot

[Morgan Fletcher]

Thank you, Simon and Kirk, for entertaining my question. I'll search for
high-resolution, local DEM data. I know lidar data is available. I'll ask
specific questions once I get there.

Morgan

On Wed, Oct 22, 2025, 8:31 AM Kirk Schmidt via QGIS-User <
qgis-user at lists.osgeo.org> wrote:

> 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
>
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