Changes between Version 8 and Version 9 of Processing/LIDARDEMs


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Timestamp:
Apr 15, 2008 4:27:06 PM (12 years ago)
Author:
benj
Comment:

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  • Processing/LIDARDEMs

    v8 v9  
    66time(gps?)(last pulse) ZZeasting northing height(m) pulse_time? (first pulse) ZZeasting northing height(m) pulse_time?
    77}}}
     8
     9== Automatic method ==
     10
     11This method runs a script file that does all the steps from the manual method below. Use it unless for some reason you can't.
     12
     131. If you don't already have one, create a GRASS location that is appropriate for the area covered by the Lidar data.
     14
     152. Run the script file
     16{{{
     17lidar2dem.sh <grass_mapset_directory> [Intermediate_file] [--reuse|--overwrite]
     18}}}
     19
     20Note that while it is not necessary to specify an intermediate file (in this case a temporary file will be used), it is recommended that you do so. This is because if you specify one then it can be reused for subsequent runs (saving time), and because the name of the intermediate file is used as a base for the file name of the GRASS map that is created and of the DEM. You may specify --reuse or --overwrite to determine what will be done if the intermediate file already exists - if you do not specify and the file already exists you will be prompted.
     21
     22Note also that it is recommended that you check the displayed upper and lower bounds against the displayed histogram to ensure that the automatically chosen values are sensible. If they are not and this causes a problem with the DEM then it will be necessary to reprocess using the manual method as below.
     23
     24== Manual method ==
    825
    9261. Merge files and strip off unnecessary UTM zone number by running trim_lidar.sh in the directory containing the lidar files:
     
    4562}}}
    4663
    47 In the given example, there are a very few values between 20-30m at the bottom end, and a big gap at the top end before a small number of values above 590m (nothing between 240m and 590m). So it would be sensible to pick 30m as a minimum cutoff and 250m as a maximum. Note that because the histogram is on a log scale the difference in numbers of points at both ends is much larger than it looks at first glance.
     64In the given example, there are a very few values between 20-30m at the bottom end, and a big gap at the top end before a small number of values above 590m (nothing between 240m and 590m). So it would be sensible to pick 30m as a minimum cutoff and 250m as a maximum. Note that because the histogram is on a log scale the difference in numbers of points at both ends is much larger than it looks at first glance. The script will display values that it thinks are appropriate cut-off values.
    4865
    49 3. Check the LIDAR data projection in the header file. Open GRASS, select (or create) an a location in the appropriate projection and set the region appropriately for the dataset - easiest way is to look at the start of the trimmed lidar data file for the starting co-ordinates of the first line, set an approximate region from there, view the map in GRASS and then adjust the region appropriately.
     663. Check the LIDAR data projection in the header file. Open GRASS, select (or create) an a location in the appropriate projection and set the region appropriately for the dataset - geographic boundaries for the data are given by the script in the previous step, though you may wish to add a small amount of padding to this to ensure that the produced DEM covers the flight area.
    5067
    51684. Read in the trimmed lidar file using r.in.xyz. Note that for large areas this will use a LOT of memory - if the command refuses to start complaining of a lack of memory, use the percent argument and the command will automatically run in several passes - this will take longer, but use less memory on each pass.