I import a 3DES IMG raster layer formatted in NAD83 metric vertical units (EPSG:26911) from the USGS 3D Elevation program. When I look at the elevation values in the layer properties, they appear to be about 1/3.2808 of the expected elevations. The layer comes in with a range of 254 to 304 meters in an area that ranges from 2732 to 3261 feet in the real world. That should translate to a range of 832-994 meters. This roughly corresponds to it converting from feet to meters, but the source is in meters already.

Is there a way to verify the source IMG values?

Is there a way to prevent this conversion (assuming the source data is valid) in QGIS?

  • If you open a new QGIS project and add the original .img file to the display, does it show wrong values when you use the identify tool? Does it show a wrong range in the layer properties? I tried with a dataset in California (one meter x47y366 CA SanDiegoQL2) and the values are indeed in meters and seem perfectly fine. Which tile/region are you working in? – FSimardGIS Feb 1 '20 at 1:36
  • Same result in a new, blank project. It's the one meter, x47y382 San Bernardino County image. – RyanJMcGowan Feb 1 '20 at 2:06
  • And the identify tool shows 200-300 values. Nothing in this region should be below 830. – RyanJMcGowan Feb 1 '20 at 2:08
  • The San Diego map seems fine. I don't have the same issue on that map. Can someone download the x47y382 map and see if you have the same issue? It may be an error at the source. – RyanJMcGowan Feb 1 '20 at 2:24
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    Yes, I get the same issue as well. Seems like the elevations are off for that tile or that project. – FSimardGIS Feb 1 '20 at 2:36

It seems that there is a problem with the elevation values in this tile and other tiles in that area of San Bernardino. I got the same issue as you describe when opening the tile, the elevation values are way off. However, tiles in other projects/cities appear perfectly fine.

If you divide the values by 0.3048 with raster calculator, the values match perfectly with the actual elevations. That could solve the issue. Perhaps a wrong unit conversion has been made somewhere when they processed the files for that project, because other tiles in the same project seem wrong too.

  • This corresponds to the conversion of meters to feet so is probably a wrong unit. – Leigh Bettenay Feb 1 '20 at 23:10

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