Creating a Topographic image

This example utilizes DEM topography data 5 minute topography data to create a topographic image that may be used as a basemap. It is similar to the Satellite Basemap example, but visualizes the topography/bathymetry digital elevation model data.

Tools Used

Gempak Programs

Data Used

Steps

  1. Create a topographic grid for desired area using GDTOPO 
      
    GDFILE   = nam_basemap.gem
    GAREA    = 10;-140;65;-40
    GDATTIM  = 070101/0000
    GVCORD   = none
    GFUNC    = topo
    TOPOFL   = dem5
    IJSKIP   = 0
    GEMPAK-GDTOPO>r


    GDTOPO PARAMETERS: 

    Grid file:       nam_basemap.gem                                                                                                         

    Topography file: /home/gempak/GEMPAK5.10.2/gempak/tables/unidata/world_topo.5min                                                                 

    GAREA:              10;-140;65;-40                              
    Lower left corner:      10.0000  -140.0000
    Upper right corner:     65.0000   -40.0000
   
    Number of X/Y grid points:   1200  660
        Number of Grid points:       792000

    Minimum and Maximum values -8103.00  4700.00

  2. Create an image from topography/bathymetry data using GD2IMG
    • Use a 2 part calibration range for band 3. Use pixel values 0 to 7 for negative values (-8103 to 0 meters), and pixel values 8 to 95 to represent positive values (0 to 4700 meters). The region selected represents a 6 Km resolution product. 
          GDATTIM  = last
    GDFILE   = nam_basemap.gem
    GLEVEL   = 0
    GVCORD   = none
    GFUNC    = topo
    SCALE    = 0
    PROJ     = str/90;-97;0
    GRDAREA  = 15;-116;50;-51.95
    KXKY     = 1002;925
    CPYFIL   =  
    SATFIL   = nam_basemap.gini
    CALINFO  = 99/3/TOPO,0,7,-8103,0;8,95,0,4700
    WMOHDR   = TICZ99/CHIZ/
    GEMPAK-GD2IMG>r
  3. Create a color lookup table representing ocean and land for the 2 ranges of data above. The color values 0 to 95 will be divided into 2 sections (0 to 7 providing shades of blue corresponding to depth below sea level, and 8 to 95 representing evelation above sea level). You can download the color table upc_dem5.tbl generated for this topographic image.
  4. Add an image type entry for this band 2**(3 - 1) image for sensor 11 (Gini ID 99 above) into GEMTBL/sat/imgtyp.tbl if it does not already exist. 
       TOPO                 TOPO_DEM5     0     94     11      4      1 upc_dem5.tbl
  5. Display the composite image in GEMPAK programs as a satellite image and overlay data as desired. 
      
    MAP      = 1
    GAREA    = dset
    PROJ     = sat
    SATFIL   = nam_basemap.gini
    LUTFIL   = upc_dem5.tbl

Part II: Mask lakes above sea level and valleys below sea level

The DEM topography/bathymetry data set used above provides elevation above and below sea level, however, it does not distinguish between land and water. The Great Lakes and Great Salt Lake represent areas above sea level that we might want to be the color of water, while Death Valley and the Salton Sea areas have areas below sea level that we would want to represent as land. If we used the 0.5 degree land/sea mask functions in GEMPAK, the boundary areas would be much coarser than the 6 Km resolution of the image product and appear blocky. We could use the land mask from one of the operational models, but to obtain the best reolution, we'll use a 1 Km (30 second) land classification data set.

  1. Create a grid from 1km land classification raster data. We'll set IJSKIP to 5 since that approximates the resolution of the image we are producing. 
           GDFILE   = landuse.gem
     GAREA    = 10;-140;65;-40
     GDATTIM  = 070101/0000
     GVCORD   = none
     GFUNC    = luse
     TOPOFL   = land1
     IJSKIP   = 5
     GEMPAK-GDTOPO>

  2. Create a boolean grid (1=land, 0=water) based on the classification type values greater than 0 representing land. Write this LUSE grid out to the nam_basemap.gem grid file created previously. 
          GDFILE    = landuse.gem
    GDOUTF   = nam_basemap.gem
    GFUNC    = bool(sgt(luse,0))
    GDATTIM  = 070101/0000
    GLEVEL   = 0
    GVCORD   = none
    GRDNAM   = luse
    GRDTYP   = S
    GPACK    =  
    GRDHDR   =  
    PROJ     =  
    GRDAREA  =  
    KXKY     =  
    MAXGRD   = 200
    CPYFIL   =  
    ANLYSS   = 4/2;2;2;2
    GEMPAK-GDDIAG>

  3. Create a new nam_topomap.gini image using the boolean land use grid to mask:
    • Water areas below 0 m (sea level)
    • Set water areas above sea level to 0 m
    • Remaining topography above sea level unchanged
    • All other points (eg land areas below sea level) assigned a value of 1 m 

          GDATTIM  = 070101/0000
    GDFILE   = nam_basemap.gem
    GLEVEL   = 0
    GVCORD   = none
    GFUNC    = miss(miss(miss(mask(slt(topo,0),slt(luse,.5)),mask(0,slt(luse,.5))),sgt(topo,0)),1)
    SCALE    = 0
    PROJ     = str/90;-97;0
    GRDAREA  = 15;-116;50;-51.95
    KXKY     = 1002;925
    CPYFIL   =  
    SATFIL   = nam_topomap.gini
    CALINFO  = 99/3/TOPO,0,7,-8103,0;8,95,0,4700
    WMOHDR   = TICZ99/CHIZ/
    GEMPAK-GD2IMG>

  4. Display the land/water masked topography image as above 
      
    MAP      = 1
    GAREA    = dset
    PROJ     = sat
    SATFIL   = nam_topomap.gini
    LUTFIL   = upc_dem5.tbl