I re-wrote this post from an original one and need new answers please!
I'm using Qgis 2.18.15 to extract 1' contours from a DEM created from my drone using photogrammetry software.
My main question is how can I reduce the error in calculating these contours?
The steps to do this:
Upload the geotiff DEM CRS ESPG:3857(meters)
Next I re-project the data to the local code EPSG: 32048 which is in feet.
(SOLUTION) Raster's (properties > styles) set Render Type: Singleband Grey. Set min and max to 0 - 255. Contrast enhancement: Stretch to MinMax I'm not sure why but I've changed the values many times and this range always seems to encompass all of the data in the DEM. Is there a reason for these values like some kind of default?
I set this to have a 0 min so the range becomes 65.29' - 7.41' = range 0 to 57.88. So for a scale factor I take 57.88 / 255 = 0.226980392156. I plug this into the raster calculator and get this:
The math here using the raster calculator does not make sense.
255 x 0.226980392156 = 57.88 not 57.8223 and 0 x 0.226980392156 = 0 not 0.457592
Continuing with the values I extract 1' contours now that it is scaled
Why does the scale factor math using the raster calculator not make sense? Also, is there a way to explain the min max range 0 to 255 that I've found works well when the DEM comes in without z values?
I need to be as accurate as possible with this data and I know there are multiple places in this process which can skew the data.