1

The contour map that I generate from a Maps Made Easy (MME) DEM produces a distorted result and I’d like to learn how to correct the problem.

Shown is a pool that has been physically measured to be rectangular. To visualize that the edges are orthogonal I’ve rotated the clip and place a rectangle over it (lower left).

process

Next, I imported the MME DEM into QGIS and generate a 5 ft interval contour map (upper right). Notice that the rectangular pool contour is a rhombus. If the result is exported as a DXF, the DXF file contains a rhombus. Oddly the MME GeoTIFF looks like a rhombus too in QGIS, but not in a TIFF viewer.

To illustrate the distortion, the QGIS contour is rotated and a rectangle has been placed over it (lower right).

I’ve changed the CRS to Google Pseudo Mercator, as recommended by MME, but the distortion does not change.

How can I eliminate the distortion?

Below is the contour line superimposed over the geoTIFF. Even the geoTIFF is distorted. enter image description here

  • I don't think you can expect a contour to be exactly at the edge of the pool. Contours represent a more generalized transition between different elevations, they don't demarcate specific topographic features. – csk Nov 5 '18 at 17:11
  • csk, thanks for your reply. Look at the contour line superimposed on the geoTIFF in QGIS above. It is also distorted. What do you make of that? – EarthCarver Nov 6 '18 at 5:25
  • Distortion is normal when viewing a georeferenced image in a GIS program. The distortion is because the image gets stretched or warped to make it line up with the coordinate reference system you're using. If that bothers you, try out different CRS's until you find one that looks right to you (just change the CRS of the project, don't bother re-projecting the geotiff). – csk Nov 6 '18 at 16:59
1

Oddly the MME GeoTIFF looks like a rhombus too in QGIS, but not in a TIFF viewer.

Obviously the projection (CRS) of the TIFF file differs from the projection (CRS) of your QGIS map canvas. To change the latter, right click the TIFF in QGIS's layer tree and select "Set CRS", "Set Project CRS from Layer".

  • Yes, like @Redoute says, it looks like you have a projection problem. I am unfamiliar with MME but maybe you had the option to define the CRS in MME and defined it wrong thus returning bad data. When you say you changed the CRS in QGIS did you change the CRS of the QGIS project or did you run Warp on the TIFF? Changing the CRS in the QGIS project could add to the distortion if the project and the data do not share the same CRS. Ensure that the data and the project share a CRS and ensure that the CRS optimizes shape. Also, was this pool in the center of your drone flight or on the edge? – GBG Nov 6 '18 at 17:04
  • Thanks, everyone. The problem was resolved by changing the QGIS project CRS to WGS 84 / Pseudo-Mercator (EPSG:3857). The pool is now rectangular. – EarthCarver Nov 7 '18 at 20:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.