Global Mapper v25.0

Making constant slope contour banks in GM

nickbart
nickbart Global Mapper User
edited February 2014 in Technical Support
Hi, Does anyone know if Global Mapper will/can produce contours/lines with a constant slope. I have built a contour map from LIDAR data, and now want to draw contour banks on a map using the elevation data. To do this I want to create a line with a constant slope of -0.025% where the line is produced from the elevation dataset. Cheers Nick

Comments

  • energy
    energy Global Mapper User Trusted User
    edited February 2013
    Hi,as far as I understand you try to create slope contour map according to percent values?
  • global_mapper
    global_mapper Administrator
    edited February 2013
    Nick,

    What you need to do is export to a Float/Grid using File->Export->Export Elevation Data and check the box to export slope values rather than elevations. Then bring that back in and generate contours from that as the reported "elevation" values will actually be slopes in degrees.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • nickbart
    nickbart Global Mapper User
    edited February 2013
    Hi Mike, Thanks for the reply, but doesn't give me the required result. Gives me a map with slope contours - I don't think I was specific enough in my original post. I was hoping to be able to draw a line from a starting point, with a set slope, to a finishing point i.e. I'd like to start from a certain elevation contour (created using LIDAR data), in a certain direction, with a certain set slope, and follow the contour across/downslope automatically, until I reach a boundary or place where I need to stop the contour bank (where the water would drain to) - make sense???? If I don't make sense, I will try to draw an example and post it Cheers,
    Nick,

    What you need to do is export to a Float/Grid using File->Export->Export Elevation Data and check the box to export slope values rather than elevations. Then bring that back in and generate contours from that as the reported "elevation" values will actually be slopes in degrees.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/
  • global_mapper
    global_mapper Administrator
    edited February 2013
    Ah that is different. For what you are describing some of the watershed functionality might be the way to go. You can create a point feature to trace flow from, then select that (you can select multiple points) with the Digitizer Tool, then use the Analysis->Generate Watershed command and select the option to trace flow from the selected point(s). You can also trace from lines if you have a linear feature to trace flow from. You will then get an arrowed line visualizing where the flow would go from that point.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.bluemarblegeo.com/products/global-mapper.php
  • Greg Nash
    Greg Nash Global Mapper User Trusted User
    edited February 2014
    check the box to export slope values rather than elevations

    I'm using Global Energy Mapper v14.2 (b052813) [64 bit]. I don't seem to have this option to export slope values rather than elevations. Am I exporting to the wrong format? Was this option created in a later release? If so, which release?

    My goal is to calculate the (horizontal) surface area where slopes are greater than or equal to fifteen degrees. I also plan to calculate the remaining area, where slopes are less than fifteen degrees.
  • global_mapper
    global_mapper Administrator
    edited February 2014
    The option to export slope values rather than elevations is only for the Float/Grid export format, but does exist in v14.2 for that format. It's not there for any other elevation formats though.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • Greg Nash
    Greg Nash Global Mapper User Trusted User
    edited February 2014
    Thanks, that worked. I was using the "Global Mapper Grid" format.
  • Greg Nash
    Greg Nash Global Mapper User Trusted User
    edited February 2014
    I'm getting a slight difference in the values after the export/import. Am I doing something wrong? The slope values seem to be changing by maybe half a degree across the entire map.

    Elevation Slopes.pngElevation Slopes Converted.png
  • global_mapper
    global_mapper Administrator
    edited February 2014
    What did you choose for the resampling option when exporting to the Float/Grid? That would affect the output Float/Grid as is would resample the slope values as well. Also does the difference appear the same when you zoom way in? You could just be seeing a blend effect when zoomed out.

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation
  • Greg Nash
    Greg Nash Global Mapper User Trusted User
    edited February 2014
    Yes, the difference stays when I zoom in.

    For the sampling, I used the default, which was a little over 30 feet. I assumed that to be about 10 meters and the elevation data was 10m data, so I figured that was a good choice. I tried resampling at a tighter interval (I don't remember what) but that seemed to increase the difference between the figures. However, I did that test when I had a foot/meter bug in my procedure so I didn't do much comparison.

    Are you having a problem with reproducing the effect?
  • global_mapper
    global_mapper Administrator
    edited February 2014
    I did the same kind of test as you and thought I was seeing the same thing, then I turned off hill shading and set the Resampling method to Nearest Neighbor (no interpolation) for the layers and then the slope-shaded terrain and the slope Float/Grid file (shaded with the "elevation" shader with the same coloring) looked identical, so it seems as though it was an effect caused by the different treatment of slope and elevation grids for resampling and shading instead of actual differences in the slope values.

    Does your data align better if you turn off resampling and hill shading?

    Thanks,

    Mike
    Global Mapper Guru
    geohelp@bluemarblegeo.com
    Blue Marble Geographics for Coordinate Conversion, Image Reprojection and Vector Translation