Monday, June 20, 2011

Graphically locate the lowest and highest DEM points in Global Mapper

Global Mapper 12 has a function to find the extreme elevation values in a digital elevation model (DEM) layer. It can only create a text file reporting on the lowest and highest elevations in a layer including the coordinates. If I want to locate the points on the map, then I will have to open up the text file and try to locate the points manually on the map. I found an alternative way to find the highest and lowest elevation points within a polygon graphically. It involves a bit of work as described below.
  1. Run Global Mapper. Load in a DEM file e.g. ground.asc.

  2. Press ALT+C to display the Control Center.
  3. In the Overlay Control Center, mouse right click on the DEM layer.

    A pop up menu appears.
  4. Choose Create Point Features at Elevation Grid Cell Centers.

    A new point layer "ground.asc Points" is created.

  5. Load in or create a polygon to define the area of interest for analysis.

  6. In the Overlay Control Center, toggle off the DEM e.g. ground.asc and the derivate point layer e.g ground.asc Points.
  7. Press ALT+D. Mouse left click on the polygon.

    The polygon is selected.
  8. Mouse right click anywhere on a blank space on the map window.

    A pop up menu appears.
  9. Choose Advanced Selection Options | Select All Point Features   within the Selected Area(s).

    The points inside the polygon are selected.
  10. Mouse right click on a blank space in the map window.

    A pop up menu appears.
  11. Choose Edit Selected Features.

    The Modify Selected Point Features dialog box appears.
  12. In the Feature Layer combo box, choose Create New Layer for Feature. Click OK.

    The Enter Layer Name dialog box appears.
  13. Type in a new layer name e.g. aoi. Click OK.

    The selected points are moved to their own layer.
  14. In the Overlay Control Center dialog box ,toggle off all the layers except the newly create layer aoi.

  15. Select Search | Search by Attributes, Names and Description.

    The Search Vector Data dialog box appears.
  16. Click the ELEVATION column header.

    The ELEVATION column is sorted in ascending order or descending order.
  17. Click on the lowest elevation record in the list to highlight the corresponding lowest elevation in the polygon on the terrain, and vice verse for the highest elevation.

    The location on the map is highlighted.


r@m said...

When I try this method I get "This operation may create over 36000000 points" warning.
Am I doing something wrong ?


d0min0c said...

Hi R@m, you aren't doing anything wrong. It's just that your grid is rather large (about 6000x6000) and the software is warning you that it will create that many points if you proceed. For such a large size grid, you might be better of using an alternative method such as resampling down your grid or generate contours.

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