Author Topic: Basic Steps to Convert 1/3 Arc DEM to Civil3D in Feet  (Read 6149 times)

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MSTG007

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Basic Steps to Convert 1/3 Arc DEM to Civil3D in Feet
« on: March 02, 2023, 08:15:44 AM »
I wanted to get your thoughts on this workflow I found. I did not know if there was a better method to do this.
I cannot seem to remember the link to where I found this, But I thought it made sense to me.

Quote
1/3 ARC DEM to Civil3D (Clipped and Feet)

<<….I go to USGS for some elevation data , I draw my polygon AOI and get a tiff of half of Long Island. Needless to say it Crushes civil3d….>

Using AutoCAD and Toolspace, you can do this:
•   Create your AOI by drawing a rectangle.
•   Specify the native coordinate system of your DEM.

If you determine the DEM’s northings/eastings and elevation is in meters and you want to convert these to feet, do this:
1.   Open brand new, clean dwg and assign NY State Plane, Long Island, NAD83, meters=>Turn on Bing Hybrid=>Draw a rectangle (as small as possible) around your AOI using Bing Hybrid as your guide. Turn off Bing Hybrid.
2.   Create and name a new surface on Prospector Tab=>Definition=>Right click Boundary=>Add=>Data Clip=>OK=>select your AOI Rectangle.
3.   Right click DEM=>Add=>at top of the ‘Add DEM File’ dialog browse to then select your DEM.
4.   While still in Add DEM dialog window, click the far right-side of row named CS Code=>enter the native coordinate system of the DEM=>click OK to establish the DEM’s coordsys. (Refer to image below.)

5.   OK to load the DEM portion confined to your rectangle (AOI). This may take a minute or two.
6.   On Toolspace right click surface=>Export to LandXML=>OK. Save the XML in your project folder. Name it ‘MyExport_meters’ so you don’t forget it was exported and has units of meters. Save and close this dwg.
7.   Open a brand new clean dwg and assign NY State Plane, NAD83, Long Island, feet.
8.   Go to Insert Tab=>Import Panel=>LandXML=>browse to and select MyExportXML_meters.xml=> Open=>OK
9.   Turn on Bing Hybrid and confirm elevations are in feet and not meters.

Typically, DEMs downloaded from USGS use either projected coordinates in UTM-Zone meters or geographic coordinates in Lat/Long degrees. It all depends on what you download (refer to image below).
 

 

Civil3D 2020

kirby

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Re: Basic Steps to Convert 1/3 Arc DEM to Civil3D in Feet
« Reply #1 on: March 03, 2023, 09:14:20 AM »
Hi MSTG007

We use Global Mapper software for this type of reprojection.
1. Import DEM (in native Lat Long)
2. Change Projection from Geographic or Mercator to UTM, change datum from Google Maps (sphere radius...) to NAD 83 (or NAD 84 CSRS in Canada), however always use NAD 83 to create Civil3D compatible DEM.
3. Export by window or polygon boundary, export to grid format (binary DEM) and select output grid x & y resolution (you have direct control over resolution, just may not make much sense to convert a ~10m dem to a 1m grid)
4. Done!

I'm from Canada, and the 1 arc-second SRTM DEM has only recently become available outside of the US.  Where do you find 1/3 arc-second DEMs? (is this the same as USGS 3DEP?)  I can't find it on the usual sources like USGS EarthExplorer.

Global Mapper can download from common online sources (or you can specify your own).  I've downloaded 3DEP DEMs but these seem to have elevations truncated to the nearest foot, so not entirely useful for hydrology.


MSTG007

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Re: Basic Steps to Convert 1/3 Arc DEM to Civil3D in Feet
« Reply #2 on: March 03, 2023, 09:19:38 AM »
I usually use this link here to download the 1/3 arc.

https://apps.nationalmap.gov/downloader/

I hope that helps some.

Civil3D 2020

kirby

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Re: Basic Steps to Convert 1/3 Arc DEM to Civil3D in Feet
« Reply #3 on: March 03, 2023, 09:41:24 AM »
Thanks, looks like it is limited to US only, with a little overspill at borders.

(coincidentally, I'm in Winnipeg just outside the data limits  :rip:)