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15.3 Masking Features at Land-Water Boundaries

McIDAS does not provide a direct way to display color-filled contours of gridded data sets. Instead, one can convert gridded fields into imagery that can then be colorized using the EU command and manipulated by a variety of imagery commands.

Many Unidata McIDAS users have asked if it is possible to clip contours of gridded fields at land-water boundaries. The ability to do this is particularly desired for gridded renditions of point source measurements due to the paucity of observations over water. If one has an image data set from which the land-water boundaries can be readily discerned, then this data set can be used to select values from one image that lie exclusively over land or sea for inclusion in a new image product.

IMGFILT generates a new image by applying a filter mask to data from an existing image or images. IMGFILT is routinely used to clean up images by applying a shot noise filter to each element and is frequently used to identify cloudy regions by comparing colocated image values from different spectral channels.

All images used in the filtering process must have the same projection, resolution and navigation. You can use the IMGCOPY or IMGREMAP before using IMGFILT.

  1. If a McIDAS session is not already running, start one with at least six frames.

    Type: mcidas

    at the Unix prompt.

Gridded Field Clipped to Eliminate Contours over Water

The first step in generating an image clipped using IMGFILT is to generate an image representation of a gridded field. Before we can do that, we need to locate a gridded field in McIDAS compatible format.

  1. Point to the cooperating ADDE server operated at UCAR (adjust the name to the server being used for workshop activities).

    Type: DATALOC ADD RTGRIBS UNI50.UCAR.EDU

  2. List the first ten gridded fields of the NAM-ALL dataset available on the ADDE server.

    Type: GRDLIST RTGRIBS/NAM-ALL NUM=10

  3. Copy one of the gridded fields to a local data set.

    Type: GRDCOPY RTGRIBS/NAM-ALL MYDATA/GRIDS.132 PAR=T LEV=1000 DGRID=1

    This copies the first 1000 mb Temperature grid found in the RTGRIBS/NAM-ALL data set to the local MYDATA/GRIDS data set in position 132. The gridded field being copied will be put into the first slot in the output dataset position by virtue of the DGRID=1 keyword specification.

    Since we defined MYDATA/GRIDS to include all possible GRID data files, position 132 will correspond to the file GRID0132 which, if it does not already exist, will be created in your current McIDAS working directory.

  4. Verify that the grid was copied to the local data set correctly.

    Type: GRDLIST MYDATA/GRIDS.132

    If the grid was copied successfully, you will see a listing something like: 

    GRDLIST MYDATA/GRIDS.132
Dataset position 132    Directory Title=

PAR    LEVEL       DAY        TIME    SRC FHOUR     FDAY       FTIME  GRID  PRO
---- --------- ------------ -------- ---- ----- ------------ -------- ----- ----
T    1000 MB   19 AUG 01231 00:00:00  NAM     0 19 AUG 01231 00:00:00     1 LAMB
Number of grids listed = 1
GRDLIST - done
  5. Convert the grid in GRID dataset MYDATA/GRIDS.132 to an image.

    Type: GRDIMG MYDATA/GRIDS.132 MYDATA/IMAGES.3020 MAG=7 7 PAR=T LEV=1000

  6. Display the resulting image and draw a map on it.

    Type: IMGDISP MYDATA/IMAGES.3020 1 SF=YES EU=IMAGE REFRESH='EG;MAP H'

    We were able to refer to the image that was created in AREA file AREA3020 as MYDATA/IMAGES.3020 by virtue of having defined the MYDATA/IMAGES data set to include all images in AREA file format.

    Note that the image representation of the gridded field extends well beyond the coastal boundaries depicted by the map that was drawn. Our next objective is to create a new image that excludes the data portions that extend over water.

  7. Make a copy of the image just created.

    Type: IMGCOPY MYDATA/IMAGES.3020 MYDATA/IMAGES.3021 SIZE=ALL

    We will use this copy to create a filter mask.

  8. Remap the global image topography data set into the copy just made.

    Type: IMGREMAP TOPO/GLOB MYDATA/IMAGES.3021 SMOOTH=YES

    We remap the topography data into the image copy since the topography data essentially has the descrimination between land and water that we seek (the land having a greater elevation than water).

  9. Display the remapped topography image and draw a map on it.

    Type: IMGDISP MYDATA/IMAGES.3021 2 SF=YES EU=TOPO REFRESH='EG;MAP H'

  10. Use IMGFILT on the remapped topography image to create an image that has only two brightness values: 0 over water, and 255 over land.

    Type: IMGFILT MYDATA/IMAGES.3021 MYDATA/IMAGES.3021 MYDATA/IMAGES.3022 FILTER=CORE 149 149 255 0

  11. Display the filter image and draw a map on it.

    Type: IMGDISP MYDATA/IMAGES.3022 3 SF=YES EU=IMAGE REFRESH='EG;MAP H'

  12. Now use IMGFILT with the filter image just created to mask out data values that lie over water in the original image created from a grid.

    Type: IMGFILT MYDATA/IMAGES.3020 MYDATA/IMAGES.3022 MYDATA/IMAGES.3033 FILTER=DIS 0 255 1 255 0

  13. Display the filtered image in frame 4 and draw a map on top of it.

    Type: IMGDISP MYDATA/IMAGES.3033 4 SF=YES EU=IMAGE REFRESH='EG;MAP H'

  14. Step through all of the images.

    Type: LB 1 4
    Press: Alt A

Visualizing model data as imagery

The process of creating a land-water clipped image representation of a grid can be automated. We have created a McBASI script - CFILLCLP.MCB which can be used to create a clipped or non-clipped representation from any gridded data field located in accessible ADDE datasets.

  1. Try creating a color filled representation of a 12 hour forecast, 850 mb NAM Temperature field displaying the result in the current frame using a colorful enhancement:

    Type: CFILLCLP RTGRIBS/NAM-ALL T 850 #Y 12 12 LAMB MYDATA/IMAGES.3020 Y Y SST
    Type: BAR


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