Continuous raster legend color tables from ps.map?
malcolm at cast.uark.edu
Mon Feb 19 07:00:00 EST 1996
On Mon, 19 Feb 1996, Bill Hargrove wrote:
> Is there a way with ps.map to do a raster color table legend for a
> continuous variable like elevation? ps.map is great, but pages and
> pages of individual values with boxes are less than useful ...
> Bill H.
Ok, I'm going to divulge one of our _secret_ techniques (with thanks to
Rob Dzur of CAST). No, there is no way to create a continuously variable
color bar directly in ps.map, but you can readily create a raster layer
that can be patched into the raster layer you will use for your map. Here
1. Let's say you're working with an (unlikely!) elevation map that has
values from 1 to 100. You will need to generate a color bar with 100 shades.
2. Determine where you want to place the color bar on your map. The
easiest way to do this is to use d.where, and find the coordinates of the
upper-left and lower-right corners of your color bar. In other words, you
can determine the width and length of the bar by picking the corners of
it on your monitor. Let's say that we get the following numbers:
Upper-left: 594700 4829100
Lower-right: 595700 4829000
So, we will be making a color bar 100 meters high by 1000 meters long.
3. Construct the header for an ASCII raster file (see man page for
r.in.ascii). The trick is in setting your rows and columns; we will
create _rectangular_ cells (not square)! In this case, since we want the
color to vary from left to right, we'll specify only _1_ row and 100
columns (the number of categories that are in are original raster layer):
4. Next, you need to enter the values for the cells. Since there will
only be one row of 100 cells, we simply need to enter one value for each
cell, 1 through 100. These can be simply typed in, or you can write a
brief shell script to increment and concatenate to the header. It doesn't
matter how many values you write on one line; GRASS looks at the header
info to determine the raster file structure.
1 2 3 4 5 6 7 etc.
5. Run r.in.ascii to load this information into a GRASS raster file;
let's call our example "colorbar".
6. Patch raster layer "colorbar" with the raster layer that it is based
upon. Make sure that the "colorbar" layer is the second map in the input
r.in.ascii input=elevation,colorbar output=elevation_plus
The output map will apply the colortable from the first input map
("elevation") to all of the raster cells, including those in the color bar.
7. In ps.map, use the "text" command to place any text that you want
associated with the color bar.
That's it! It's pretty easy after you try it once.
Malcolm D. Williamson - GIS Specialist E-mail: malcolm at cast.uark.edu
Center for Advanced Spatial Technologies Telephone: (501) 575-6159
Ozark Rm. 12 Fax: (501) 575-5218
University of Arkansas
Fayetteville, AR 72701
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