Issues getting accurate contour data from USGS GEOtiff

[ericL4ECC]

I'm looking to learn this process myself and am having issues. I have a GEOtiff sourced from USGS that a colleague gave me that he used to create the accurate topo for a site (red and blue 2' and 10' contours in the image). I attempted to make an expanded version by creating a surface from DEM and using a data clip, as the file is large compared to the site I need.

I've tried this countless different ways and means and even got the data from another source and I ended up with consistent errors each time:

- elevations in the 100's when the actual elevation of the site is in the 500's 

- obviously inaccurate contour intervals (as you can see in the image the actual 2' intervals are way tighter than mine)

I've exhausted all options I can think of, does anyone have any idea what I am doing wrong?  

Link to download USGS file: https://prd-tnm.s3.amazonaws.com/StagedProducts/Elevation/1m/Projects/TX_Neches_B4_2016/TIFF/USGS_on...

[ChrisRS · Accepted solution]

I suspect that the data is metric and you are bringing it in as feet. 

[ChicagoLooper · Accepted solution]

Nacogdoches, correct? In Garrison?

You need to consider the original units of the data (UTM83-15). The elevation data (the z-value) is meters too, not feet. The elevation units you’re viewing in your uploaded image are meters. 

BTW, the contours generated in the image look jagged and have a zig-zag appearance. Using the same tiff elevation data and the AutoCAD Map3D toolset, you can also generate smoother, more spline-like contours, typical like the ones you see in topographic maps. 

<<…I've tried this countless different ways and means…>>

Hmmm…there are more ways than you think. 

[ericL4ECC]

It's always the simplest answer, I spun my wheels all day yesterday trying to figure that out. Thank you so much. 

I will also follow your advice and smooth those contour lines as well but I've already found how to do that. Again thank you!

[ericL4ECC]

You are 100% correct, thank you! 

[Pointdump]

Hi Eric,
When you add that big ol'(Texas talk) Tiff to your Surface Definition, be sure to tell C3D what coordinate system the DEM is in. Civil 3D is smart enough to figure out the units.
Dave

[ericL4ECC]

Thanks Dave, I was able to get that TIFF file into Civil 3d but I ended up converting the units and coordinate system to what I needed via QGISm, which in hindsight appears to be a roundabout way to get it done. For whatever reason when I was in the 'add DEM file' dialog box in your first image my drawing settings all populated but the DEM file CS code, description and so on did not populate.

Do you have any idea why that would be? 

Thanks for your help and insight, I'm amazed at how quick and helpful this forum is. 

[Pointdump]

Eric,
Click in the box to the right of "CS Code" until you see the ellipsis. Then clicking on the ellipsis will allow you to assign a coordinate system to your DEM file.
And yes, QGIS is a must have.
Dave

[ericL4ECC]

Ahh I see, that makes sense. I'm venturing into some new territory here, thanks for all your help!