Terrain flattening

ic3wallic3wall Global Mapper UserPosts: 16
edited December 2012 in Elevation Data
I have an elevation map in xyz format, I would like to be able to select a certain area and bring down all elevations to a certain value. I would also like a smooth transition with the surrounding elevations ... is that possible in GM?

I tried selecting an area and then adding an elevation attribute (like 0m) in order to flatten this area with the "Create/Flatten terrain form selected area feature" option, but no success... It looks like it flattened some spots of the area, but definately didn't bring down all elevations to the prescribed elevation attribute.

Thank you

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2012
    What version of Global Mapper are you using? What you specified that you did (i.e. add an elevation attribute and then use create/flatten area to flatten the terrain) is exactly what I would suggest. Do make sure that the option to flatten areas is checked on the gridding dialog, but that should be checked by default.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    version 13.00

    Ok I found how to flatten a certain area, but I still don't know how to make the transitions smooth with the surrounding elevations... is there a feature to do this ?

    Thank you
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2012
    Once you have the terrain flattened, keep the area selected, then go to the Feathering tab of the Options dialog for the new elevation layer. Specify to feather to the selected polygon and provide a pretty small feather distance, probably between 5 and 10 samples. This is the transition distance. You can tweak it if you need more or less transition.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    Alright, that works fine!

    However, the feathering distance is calculated in pixels, how can I relate that to meters ?

    Thank you
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2012
    It depends on the sample spacing of your grid layer. For example if it is 10 meters per sample, then a 5 pixel feather will mean feathering over 50 meters distance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    Cool, thank you so much!
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    ic3wall wrote: »
    Cool, thank you so much!

    Ok I thought everything worked fine because I only tested the feathering feature in the same orientation as my grid... but the result is not so nice when I try to feather in a different orrientation. The picture below shows the results once converted to an stl. The picture with the smooth result is when feathering is done in the same orientation as my grid, the second picture shows the results when feathering is not aligned with the grid. I tried increasing the resolution to 10m*10m when saving it, it doesn't help very much, please help..
    Capture-1.pngCapture.png
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2012
    Is the issue the stair-stepped appearance along the edges? If you go to the metadata for your custom layer in Global Mapper what is shown as the resolution for that layer (i.e. pixel size in meters)?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    yes the issue is the stair-stepped appearance, I would like that part to be as smooth as possible.

    Here's what I see in metabase:

    FILENAME=
    DESCRIPTION=smooth_box (Triangulated Elevation Grid)
    UPPER LEFT X=374629.000
    UPPER LEFT Y=5441547.000
    LOWER RIGHT X=380531.000
    LOWER RIGHT Y=5436142.000
    WEST LONGITUDE=64° 43' 4.8877" W
    NORTH LATITUDE=49° 06' 54.5339" N
    EAST LONGITUDE=64° 38' 8.1022" W
    SOUTH LATITUDE=49° 03' 55.3484" N
    PROJ_DESC=UTM Zone 20 / NAD83 / meters
    PROJ_DATUM=NAD83
    PROJ_UNITS=meters
    EPSG_CODE=26920
    COVERED AREA=31.9 sq km
    NUM COLUMNS=128
    NUM ROWS=128
    PIXEL WIDTH=46.472 meters
    PIXEL HEIGHT=42.559 meters
    MIN ELEVATION=140 meters
    MAX ELEVATION=140 meters
    ELEVATION UNITS=meters
    BIT_DEPTH=24
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2012
    I think that I found the problem. The feathering applied would only be in steps up to the pixel distance specified. Normally for imagery where you might blend over hundreds of pixels this isn't an issue because each step was less than 1% so you don't even notice, but for a low resolution elevation surface where you just blend of 10 pixels or so you are getting 10% steps in feathering as your distance was going in jumps of 1 pixel distance. I have updated this to use floating point values so it should be much smoother now on export. I have placed a new build at http://www.globalmapper.com/global_mapper13.zip with the change for you to try. Simply download that file and extract the contents into your existing v13.xx installation folder to give it a try. If you are using the 64-bit v13 version there is a new build at http://www.globalmapper.com/global_mapper13_64bit.zip .

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    Thanks for the quick reply, I just tested the file you posted. The result is slightly better but the stair-step effect is still visible. See below:Screenshot.png

    One thing I still don't understand is that the feathering tool works very good when it is used in the same direction as the grid, regardless of the pixel size ( see image posted in my previous post). So that makes me think that the problem is related to interpolation.

    Would it be possible to rotate my grid in a given direction and then apply feathering ?

    The map I'm working on is for CFD simulations, so topography is very important since it will have an effect on the flow.
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited February 2012
    The orientation shouldn't matter as far as Global Mapper is concerned, but maybe it is causing a shading effect in the other software where what are actually very small changes in elevation are getting shaded and making them look significant. One thing to try is making your grid sample spacing smaller when flattening the are to make the terrain to see if that makes a difference.

    Something else to do is export to a normal gridded format instead of STL and pull that back into Global Mapper to see if the grid itself has a stair-step or if it's just a rendering effect from the angles in the other software.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited February 2012
    The orientation shouldn't matter as far as Global Mapper is concerned, but maybe it is causing a shading effect in the other software where what are actually very small changes in elevation are getting shaded and making them look significant. One thing to try is making your grid sample spacing smaller when flattening the are to make the terrain to see if that makes a difference.

    You were right, making the grid sample smaller when flattening is the key. I'm getting much better results now, I brought the resolution down to 1m and here's the results:

    Capture.png

    Again, thank you very much!
    Pretty good support!
  • ic3wallic3wall Global Mapper User Posts: 16
    edited December 2012
    I'm sorry to bring back this old thread...
    I want the achieve exactly what I was doing but using range rings (see picture below)
    I want a smooth transition between inner circle and outer ring, I don't mind what's outside the outer ring. I'm working on an elevation map. I got the outer ring to have elevation=0m by using an attribute elevation.GM_smoothRing.png
  • global_mapperglobal_mapper Administrator Posts: 17,238
    edited December 2012
    So you want the elevation to blend from the inner ring elevations smoothly to those on the outer edge? If so try having a polygon that is just the inner polygon and then select it and go to the Feathering tab of the Options dialog for the top elevation layer and choose to feather to the selected area. Check the box to feather outside the polygon rather than inside and specify a feather distance large enough to cover the size of the ring.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ic3wallic3wall Global Mapper User Posts: 16
    edited December 2012
    Thank you I'll give it a try.
  • imagecatcherimagecatcher Global Mapper User Posts: 13
    Thought I'd keep this thread going lol. I have spent all day trying to flatten some quarry dams. My method so far:
    Created a contour at a fixed elev based on an observed waterline elev, lets say 250m. Converted the contour to both a line & area feature with all vertices at 250.
    Right click bring up Terrain - create/flatten but the box "Flatten 3D area features" is greyed out.
    Forging ahead gives me a strange grid with tin sections above the set elev. (id send an image but doing this post on a computer with image issues)
    Thanks
  • imagecatcherimagecatcher Global Mapper User Posts: 13
    Thought I'd keep this thread going lol. I have spent all day trying to flatten some quarry dams. My method so far:
    Created a contour at a fixed elev based on an observed waterline elev, lets say 250m. Converted the contour to both a line & area feature with all vertices at 250.
    Right click bring up Terrain - create/flatten but the box "Flatten 3D area features" is greyed out.
    Forging ahead gives me a strange grid with tin sections above the set elev. (id send an image but doing this post on a computer with image issues)
    Thanks
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