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Combining DEM + ASCII?

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#1
paulojul

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Hello to all of you,

I hope you guys can give me advice. I am trying to make a more detailed DEM than the one we currently have.

Basically I have points taken from Total Station and survey-grade GPS and put those in Global Mapper as a dxf after correcting the errors in Autocad. I then exported the points as a simple ASCII from Global Mapper. Is there some way to combine that ASCII file with the existing DEM that I have? Creating a DEM from the ASCII alone is not detailed enough so I am hoping to use it instead to "update" the DEM that we have.

As for software, all I have available to me right now is Autocad, Global Mapper, Surfer, Arc GIS, Surfer, and Visual Nature Studio.

Any advice would be greatly appreciated.

-julius

#2
Hans van der Maarel

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I hope you guys can give me advice. I am trying to make a more detailed DEM than the one we currently have.

Basically I have points taken from Total Station and survey-grade GPS and put those in Global Mapper as a dxf after correcting the errors in Autocad. I then exported the points as a simple ASCII from Global Mapper. Is there some way to combine that ASCII file with the existing DEM that I have? Creating a DEM from the ASCII alone is not detailed enough so I am hoping to use it instead to "update" the DEM that we have.

As for software, all I have available to me right now is Autocad, Global Mapper, Surfer, Arc GIS, Surfer, and Visual Nature Studio.


You should be able to do that in Global Mapper. I don't know the exact steps off the top of my head, but if you get your original DEM in as control points and then your surveyed points as well, you can build a new DEM out of the combined point cloud.
Hans van der Maarel - Cartotalk Editor
Red Geographics
Email: hans@redgeographics.com / Twitter: @redgeographics

#3
canvas101

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Hello Julius,

As Hans van der Maarel mentioned you can easily do this in GlobalMapper and not so easily in AutoCAD Civil 3D. I have done this when needing to correct excessive error in DEM's with field collected data.

What is the final product you are attempting to generate (DEM or TIN)? My opinion is that if you are ultimately going to output a TIN then AutoCAD would offer some advantages, however if your final product is a DEM GlobalMapper would be easier.

Please keep in mind that DEM’s are generated by defined sample spacing (e.g. 5 meters, 1 foot, 0.333333 arc seconds, etc…) and because of this you could take high resolution data and dumb it down by the methods you employ. I hope I have provided a little insight into this and would be happy to provide my process for GlobalMapper work flow if you need it. Respond back and let me know.

Regards

#4
paulojul

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Thank you Hans and Canvas,

My final output would be a DEM.
Canvas, if you could be give me an overview of your process in Global Mapper, it would be greatly appreciated.

-julius

#5
canvas101

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Thank you Hans and Canvas,

My final output would be a DEM.
Canvas, if you could be give me an overview of your process in Global Mapper, it would be greatly appreciated.

-julius

Hello Julius,

I will give you a brief overview and if you need further explanation I will be happy to provide. I'm going to assume that your base data is a DEM and your enhanced data is in ASCII format.

That being said, first import your DEM data and then import your ASCII data (FILE>OPEN GENERIC ASCII TEXT FILE). You will then be prompted with a menu box and you need to insure that you select the appropriate selections (e.g. IMPORT TYPE>ELEVATION GRID FROM 3D POINT DATA, COORDINATE COLUMN ORDER>X/EASTING/LONGITUDE COORDINATE FIRST{typical but check}, COORDINATE DELIMETER>AUTO{once again typical but you may need to check}, CHECK MARK>TREAT 3RD COORDINATE VALUE AS ELEVATION).

Next after all settings have been verified select OK. The next menu will appear and this is where you will determine the sampling of the data to be imported. It’s very important to understand that this dialog will ultimately determine the look of your data (e.g. VERTICAL UNITS>METERS{select the native elevation of your ascii data}, GRID SPACING>MANUALLY{"SAMPLING" select the level of detail you want in the file units ~e.g. meters, feet, arc sec~}, ELEVATION GRID DISTANCE CRITERIA>LOOSE, click OK, on the next menu if both imported files are in the same projection you may be able to just click OK, if not select the projection for the ascii file).

Once those files are imported you should see the typical DEM and your enhanced ASCII file. You may notice that the smoothness of the DEM may not match the ASCII and this is due to the variance in sampling space between the two files. Next you will need to open the OVERLAY CONTROL CENTER and highlight the ASCII file and click OPTIONS. Once you are inside this menu select the FEATHER tab. This tab will allow you to merge the two files together with a seamless effect. Select FEATHER ALONG ONE OR MORE EDGES and play with the BORDER SIZE IN PIXELS to arrive at your desired look.

That was about as brief as I could make it. There are several options that I did not touch on because I don’t know enough about your project but you should have the basics to get started and as I mentioned if you need further support don’t hesitate to ask. Either way let me know how you make out.

Regards

The three attachments should give you an idea of what your finished results can look like. The data used to enhance the DEM was 10ft resolution as opposed to the 10m DEM.

Attached Files



#6
CGIS

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I'm not going into Canvas101's technique, since I'm not experienced with Global Mapper, however; you need to consider the sources of data here:

SRTM DEM - What's the vertical accuracy? Low
Total Station - Vertical accuracy is high
GPS - Handheld, isn't that hot.

You need to almost rank the inputs, in the order:
  • Total Station
  • GPS
  • DEM (SRTM, etc., all have some ambiguity in the Z)
The generic GIS approach I'd take is (this would work fine in Civil3D, Manifold, ArcGIS, etc.):

I'd plot the TS points only, create a concave hull, buffer the hull by the average horizontal separation of the TS points. Then, use the buffered hull to delete those points from the GPS point set that fall within it, merge the TS and GPS (which now no longer conflict), create a new GPS/TS hull, use the buffered hull to delete those points from the DEM point set that fall within it(and yes, I'd convert the DEM to points for this step, rather than work as a raster) that fall within it.

Now, you should a have a continuous "fabric" of DEM/GPS/TS that don't spatially conflict. Now, create a DEM at the original horizontal resolution of the DEM (90m pixels?) During the DEM creation, you'll sample the surface to average the height of the TS points that all fall within the pixel extents.

#7
paulojul

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Thank you both for replying.

I will try and experiment with both methods to see which way is the best. Thanks for the great advice.




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