Issue with geotiff elevation showing in meters, everything else is in feet and works

[ericL4ECC]

I have downloaded a geotiff to use for creating a surface (source: https://coloradohazardmapping.com/lidarDownload ) which is in the dropbox link. I then reprojected that tile using QGIS into NAD83 Colorado state planes - central zone - us ft (attached). I then imported that geotiff into my drawing (also attached)) that is set in feet and in the same coordinate system, It dropped in right where it should but was showing elevations just above 3,000', when they should be just above 10,000' (so the conversion of meters to feet I assume). 

I tried to use the raster calculator in QGIS to convert to feet but that appears to multiply everything by that number and not just the elevation info. Also tried to create a blank surface then add the DEM as well as using the 'create surface from DEM' option.

Anyone know where I went wrong? Link for original geotiff file: https://www.dropbox.com/scl/fi/ek7xd8902lvx832chxynt/CO_ArkansasValley_2010_000257.img?rlkey=va6l63x...

Really appreciate the help and insgiht!

[Pointdump]

Hi Eric,
Thanks for posting your data and the link.
You definitely want to add the DEM to a TIN Surface. The "Create Surface from DEM" doesn't allow any transformation, and because it produces a Grid Surface, only allows adding raster data. No breaklines, points or vector data.
Just curious, where did you get the .img file? The link you posted has both Tiff and LAZ files, both in the Metric EPSG:6342 NAD83(2011)/UTM zone 13N. The Tiff files can be added directly to your C3D drawing. The LAZ file will need to be run through ReCap first, but will produce excellent ground data for your surface.
Whenever I see a Tiff with mismatched units, I've found that, amazingly enough, Civil 3D is smart enough to produce a correct surface.
Dave

[ericL4ECC]

Hello Dave, appreciate the response. I've attached a  couple screengrabs from how I got the .img file. I'm unsure of how best to navigate that site but I just used the interactive map to find my area of interest and selected 'Lake County' and then 'show tiles' and chose my desired tile. I then checked DEM for format and that's how I got the .img file. 

I tried your suggestion of creating the TIN surface and then adding the DEM file afterwards but it still shows elevations in the 3,000's. 

I also tried downloading via the 'point cloud' format but got a las file. I'm unfamiliar with the process of using ReCap (or even what that program is) so a TIFF file would be much preferred. How did you download the TIFF version? Perhaps the issue is coming from the fact that I am starting with the .img file?

Really appreciate your help!

[ericL4ECC]

Believe I may have answered my own question re: how to get the TIFF files. I was unaware there were so many different data sets from different times and just went with the first one I found. I have a version that is TIFF's so hopefully all will be well now.

[Pointdump]

Eric,
There's a problem with the Tiff's. They are DEM, so buildings and trees are included. Grab the LAZ point clouds and you can filter ground points.
Here's my workflow:
1. Download 4 LAZ files of your Area of Interest.
2. Filter Ground Points and Merge, using Lastools.(Free)
3. Run that point cloud through ReCap, transforming from UTM83(2011)-13N to CO83-CF.
4. Insert the ReCap point cloud into Civil 3D and create a surface using just enough of the points to make sure the surface is built with less than 2 million points. (no MMS file gets created)

WeTransfer link:
https://we.tl/t-ePtXvhsPES
Dave

[ChicagoLooper · Accepted solution]

@ericL4ECC 

<<...I then reprojected that tile using QGIS into NAD83 Colorado state planes - central zone...>>

So you re-projected from ? ? ? ? ? to Colorado State Plane, NAD83, Central Zone, Feet. Your data, prior to reprojection, had a Native Projection, correct? What was that native projection? Was the original native projection UTM84-13N, WGS84 datum, Zone 13N, meters, or EPSG 32613? If it was, then KEEP that projection native.

For example, assume the native CS is UTM84, WGS84, Zone 13N, meters. Instead of re-projecting in QGIS to CO83-CF, don't let QGIS reproject. Instead, keep the native and let Civil3D perform the re-projection for you. When it reprojects, C3D will TRANSFORM from UTM coordinates to State Plane coordinates....and that means it'll accurately land in modelspace.

As it is now, your North Ridge - tile.tif doesn't have 'elevation units' and appears to be defaulting to elevation of meters.

Image-1

Assuming the native projection of your data is UTM84-13N, WGS84 datum, meters, Civil3D's Add DEM window would look like this:

Image-2

Since 'feet' is indicated under elevation units in Image-2, you'll have the correct elevation units (feet, not meters) when you build your surface and STILL have geospatial accuracy in the X and Y coordinates.

[Pointdump]

Eric,
Good workflow on using DEM's from @ChicagoLooper.
I see now where you got the .img file. Sadly, Civil 3D can't import that format. If you grab the 4 Tiff's covering your Area of Interest, you'll get a surface of 4 million points. (4 files of 1,000 X 1,000 pixels) Civil 3D will want to save the surface definition as a huge MMS file. That's why I prefer using Point Clouds. You can decimate the number of points as you're importing into Civil 3D to a manageable number.(less than 2million pts)
Dave

[Pointdump]

Eric,
I was nosing around and saw a cool way to convert a LAZ point file to a DEM. Video >>>Here<<<.
The resultant Tiff will have some zero elevation pixels, so in your Surface Build be sure to Exclude Elevations less than 5'
Dave

[ericL4ECC]

Thank you so much chicagolooper, that worked perfectly. Your explanation makes perfect sense as well. Really appreciate your time and assistance!