Global Mapper Pro

How to delimit depressions ?

TeeGee
TeeGee Global Mapper UserTrusted User
edited December 2011 in Elevation Data
Hi Mike,

I have an accurate DEM of a terrain with some slope, where are a lot of depressions. In fact, this terrain looks like if it was bombed.
Is there an easy way to delimit each crater, I mean finding closed contours delimiting a hole ?

Thanks,

TeeGee

Comments

  • Ice Age Mark
    Ice Age Mark Global Mapper User Trusted User
    edited July 2011
    Mike,

    Just thought I would chime in here. I don't know how/if this function could ever work, but it would sure be a useful addition. I study areas that were once glaciated, so closed depressions are frequent and abundant. I usually have to open a USGS 7.5 topo in a separate instance of GM and tediously identify each depressed area. When I generate contour intervals less that of the USGS topo, there are often additional closed contours I must hunt down also. At least sometimes I can edit the "whole" depression at once because I can select all of the contours in it at once with the digitizer tool. When doing this I usually put the selected depression contours on a separate layer as long as I'm editing anyway. This makes visually checking for all of them with the 7.5 topo easier , and also editing them all together to the barbed closed contour line type. Not a fun process. Anyway, it sure would be nice!

    That's my two bits,

    Mark
  • global_mapper
    global_mapper Administrator
    edited July 2011
    One possibility would be to select all of the lines with Digitizer Tool then right-click and select to add feature measures to them. You could then find those with a short LENGTH attribute examine those. Another option would be for me to add an option to contour generation to add a CLOSED_CONTOUR attribute to those contours that are closed, then you could search on those and convert them to area features, then add feature measurements to those and delete those below a certain area.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • TeeGee
    TeeGee Global Mapper User Trusted User
    edited July 2011
    Mike,

    I thought that this feature already exists (when generating watershed, I'm asked to 'specify the maximum depth of depression in the terrain data that will be filled') but not directly accessible. Anyway, generating watershed helps to find depressions, as by judicious choice of this filling depth, it generates a stream that ends up at the bottom of the hole. But the whole operation can take a pretty long while, and works well only if all the depressions have more or less the same depth.
    What I was thinking about is a function so that : given a closed contour line delimiting an area inside the map bounds I'm working on, the next contour line found inside this area indicates whether the slope is ascending or descending. In other terms, if I have probably a hill or probably a hole (I say probably, because with just two contour lines, you can't tell if the hill is a volcanoe with a crater at the top, or a meteoritic impact with a peak in the middle of the depression - or a meteoritic impact at the top of a hill...).
    Well, that seems easy to me, but I have just no idea of the complexity of such a function.

    Thanks,

    TeeGee
  • Ice Age Mark
    Ice Age Mark Global Mapper User Trusted User
    edited July 2011
    FWIW

    I like the idea of adding a closed contour attribute, but I don't understand how one would distinguish a hilltop with closed contours from a depression in a search. It would at least help to insure that all of them were found. Perhaps they could be made to show on the map when you highlight on the search list, like how a point does in the vertex list, to make it easier to find, check and edit (or not) the closed ones. I too thought the watershed depression filling process might be somehow modified/used for this purpose, but since you didn't mention it, perhaps not. (I know there are issues with this process that you are working to resolve.) An interesting problem, I'll keep thinking about it. Thanks for looking into it Mike. I think it would be a great feature.

    Mark
  • global_mapper
    global_mapper Administrator
    edited July 2011
    The depression filling in watershed generation is way too slow and cumbersome to be used in this manner, it involves building an entire terrain grid for the area of interest, then calculating flow directions to determine where flow piles up, then artificially increasing that elevation and calculating flow directions again to see if flow is restored, then repeating. This takes a while and wouldn't help with your problem.

    I can see that the CLOSED_CONTOUR attrbute wouldn't really help either by itself, it would have to be paired with something to check heights inside to make sure that (a) there are no lower contours inside the closed one and (b) the elevations inside the closed contour are smaller than the contour. This I think would involve a bit of work on my end.

    Thanks,

    Mike
  • TeeGee
    TeeGee Global Mapper User Trusted User
    edited July 2011
    Mike,

    Would it be easier to put on each contour line, instead of the elevation label, an arrow indicating the direction to the next lower elevation contour ? This arrow would be just perpendicular to the contour line it is attached to, and would provide a good help to find depressions.

    Thanks,

    TeeGee
  • global_mapper
    global_mapper Administrator
    edited July 2011
    TeeGee,

    That wouldn't help so much with automated depression finding though, correct? I assume you would want the final depression contours automatically found somehow and removed if below some size?

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • TeeGee
    TeeGee Global Mapper User Trusted User
    edited July 2011
    Mike,

    What I proposed in my last post was just a visual help to find depressions in a terrain looking like an egg carton : the contour map of such a terrain is a juxtaposition of closed contours, some delimiting depressions, other mounds. Finding depressions in this context is a little bit tedious, as you need to examinate carefully the elevations of the successive closed contours. With arrows showing the direction of the slope, it would be easier : converging arrows inside a closed contour show a hole, diverging arrows show an hill.

    But of course an automatic depression finding would be a Rolls Royce to me ! I don't need to remove any depression contour automatically found. Instead, as these depressions can (theorically) be filled with water, I will give a blue color to the contour lines inside the depression, including the first one delineating more or less the limit of an overflow.

    Thanks,

    TeeGee
  • global_mapper
    global_mapper Administrator
    edited July 2011
    TeeGee,

    I'll see if I can figure out some way to easily add arrows indicating the direction of the contours when generating them.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • TeeGee
    TeeGee Global Mapper User Trusted User
    edited July 2011
    Thanks a lot, Mike. I realize that my problem is not a current one, but at least are we two, with Ice Age Mark. This feature is certainly not a fundamental one, but in addition to solve my problem, I hope that it will contribute to enhance the reputation of perfect Swiss Army Knife that GM already has in mapping software.

    Thanks,

    TeeGee
  • sfturner
    sfturner Global Mapper User
    edited December 2011
    I too would love to have a feature that allows me to find depressions. Caving is all about depressions. Anyway, I do know of a convoluted way to find depressions located in grayscale images.

    First, I open my elevation data in Global Mapper. I then export this elevation data as a grayscale, geoTIFF. The geoTIFF is shaded so that lighter values represent lower elevations and darker values represent higher elevations. When I export this geoTIFF, I create a TIFF World File at the same time. Depressions in this geoTIFF would show up as lighter pixels surrounded by darker pixels.

    I then open this geoTIFF with photo editing software. I use a free program called ImageJ. ImageJ has a tool called FIND MAXIMA. FIND MAXIMA locates all of the pixels that are local maximums or peaks. Lighter pixels have higher values than darker pixels. So, FIND MAXIMA will locate the lighter pixels surrounded by darker pixels. So, finding the local maximums is equivalent to finding the depressions.

    ImageJ's FIND MAXIMA tool has various settings; so, one can decide how precise the program will search (This is like deciding on the right interval to create contour lines at in Global Mapper). Running FIND MAXIMA can create a black and white TIFF. The maximums are represented as black dots on a white background. The resultant image that is produced by ImageJ will have the same number of pixels and dimensions as the original geoTIFF. So, rename the TIFF World File that was produced earlier by giving it the same name as the image that was just exported from ImageJ. This will allow Global Mapper to place the resulting TIFF in the correct location on the map. In Global Mapper, set this TIFF's transparent color to white and overlay the image over your other layers.

    This process can even be taken one step further by importing the points into Global Mapper as a Shapefile. Since the file exported in ImageJ is pure black and pure white only, various computer programs can easily and accurately vectorize the image. With a few more tricks, one can transform the newly created vectors into Shapefiles. Adobe Illustrator, with the Mapublisher plugin, can be used to accomplish this extra step.


    find maxima.JPG
    Grayscale geoTIFF Exported from Global Mapper. The red lines represent a square mile parcel of land.

    grayscale geoTIFF exported from Global Mapper.JPG
    Yellow boxes represent depressions found using ImageJ (The yellow boxes near the red lines are errors created by the differences between the gray and red colors. If the red lines were not present, then these yellow boxes would not have been produced by ImageJ)

    3d.JPG
    A 3D view. Look closely for the tiny, blue lines. These lines are where ImageJ marked depressions.
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