Hack 75. Share Your GRASS Maps with the World

Export your carefully crafted GIS layers into human-compatible formats.

If you've worked through the GRASS hacks in this book, you might have noticed a standard finale: "Now see [Hack #75] to export your maps to an image file." Having got this far, congratulations! Now the world can know of your triumphant ascent of the GRASS learning curve.

The r.out set of GRASS functions provides many ways to export your raster layers into different GIS-compatible formats, such as ESRI's ArcInfo, binary vector format, etc. However, the most useful formats to share map data inPNG for viewing on the Web, or PostScript for print publicationboth involve some fiddling.

6.13.1. Publish Raster Maps as PNG Images

In the simplest case, you might just have a raster layer in GRASS that you want to export as a PNG image. r.out.png is ideal for these purposes:

GRASS:~ > r.out.png in=nations out=nations.png
rows = 2250, cols = 4500
Converting nations... 96%
Done.

You should note that r.out.png, like most GRASS raster functions, only operates on the current region. Be sure to run g.region rast=... first if you want to export the whole raster layer.

6.13.2. Publish PNG Images via the Display Monitor

If you have a whole set of different layers you want to export together, it's better to use the PNG display driver that comes with GRASS. This works just like writing to an X monitor with d.mon, except that it's writing directly to a .png file.

GRASS:~ > d.mon start=PNG
PNG: GRASS_TRUECOLOR status: FALSE
PNG: collecting to file: map.png,
 GRASS_WIDTH=640, GRASS_HEIGHT=480
Graphics driver [PNG] started
GRASS:~ > d.rast eco
 100%
GRASS:~ > d.vect borders type=boundary color=black
GRASS:~ > d.mon stop=PNG
Monitor 'PNG' terminated

The disadvantage of this approach is that you can't see the image while you're creating it, and it can be annoying to have to repeat a long sequence of command-line actions if you make a small mistake. This is where d.save will save you.

As discussed in [Hack #69], d.save -c prints the list of commands that were used to generate the current contents of the display monitor to its standard output. You can use shell redirection to pipe this list to a file and then manually add the d.mon start=png line to the beginning and the d.mon stop=png line to the end. The result is a shell script that will generate your PNG. You can run it directly from the GRASS shell and tweak the lines until you're quite happy with the results.

By default, the PNG monitor generates a 256-color image 640 pixels wide and 480 pixels high and stores it in a file called map.png, but we can alter that outcome with a variety of environment variables. The following commands export the map described in [Hack #69] with a black background to an 800 600 pixel True Color PNG file called western_europe.png:

GRASS:~ > export GRASS_WIDTH=800
GRASS:~ > export GRASS_HEIGHT=600
GRASS:~ > export GRASS_TRUECOLOR=TRUE
GRASS:~ > export GRASS_BACKGROUNDCOLOR=000000
GRASS:~ > export GRASS_PNGFILE=western_europe.png
GRASS:~ > d.mon start=PNG
PNG: GRASS_TRUECOLOR status: TRUE
PNG: collecting to file: western_europe.png,
 GRASS_WIDTH=800, GRASS_HEIGHT=600
 Graphics driver [PNG] started
GRASS:~ > sh ./europe.sh 
 100%
GRASS:~ > d.mon stop=PNG
Monitor 'PNG' terminated
GRASS:~ > d.mon select=x0

The last d.mon command reselects our original X11 monitor, so that any further display commands are sent there, rather than to the now-closed PNG monitor. You can now view the PNG file in an ordinary image viewer or upload it to the Web to share with your friends and colleagues.

You will probably find that, on the whole, the images output by the GRASS PNG monitor look a lot better with GRASS_TRUECOLOR set to TRUE.

 

6.13.3. Publish GRASS Maps as PostScript Files

Generating PostScript images from GRASS isn't quite as straightforward as generating PNGs, but the results will often look a lot nicer. The ps.map utility provided for this purpose is classic GRASS: it's flexible, powerful, and utterly arcane. ps.map is driven by a "little language" for display scripting that, when fed to GRASS, will generate PostScript code. With ps.map, you can add color tables, legends, custom fonts, and scale indicators, all the visual trappings of professional looking print cartography.

Figure 6-46 shows a PostScript map of selected nodes from SFLan, a community wireless network based in San Francisco. The map was made with TIGER/Line street data, National Elevation Data obtained from the USGS SDDS, and, of course, ps.map. The contour lines were generated in the fashion described in [Hack #73] .

Figure 6-46. A map of San Francisco generated with ps.map

Example 6-1 shows the set of ps.map commands used to tell GRASS to generate Figure 6-46.

Example 6-1. sf_tiger.txt: ps.map commands used to generate Figure 6-46

 raster dem

 

 vpoints sflan

 label SFLan nodes

 color blue

 fcolor 64 64 128

 symbol basic/circle

 size 10

 end

 

 vlines tiger

 where CFCC ~ 'A1'

 label Freeways

 color red

 width 1

 hcolor black

 hwidth 1

 end

 

 vlines tiger

 where CFCC ~ 'A3' OR CFCC ~ 'A2'

 label Major streets

 color white

 width 1

 hcolor red

 hwidth 1

 end

 

 vlines tiger

 where CFCC ~ 'A5' or CFCC ~ 'A4'

 label Streets

 color grey

 width 1 

 end

 

 vlines sfcontour

 label Contour lines

 color 64 64 0

 end

 

 vlegend 

 where 6 0

 fontsize 16

 end

 

 mapinfo

 where 0 0

 fontsize 16

 end

 

 header

 file sf_tiger_hdr.txt

 fontsize 28

 end

The raster command in line 1 of sf_tiger.txt draws the digital elevation model of San Francisco as a raster base layer, which had been previously colored to distinguish land and water areas. Lines 3-9 draw the location of SFLan nodes from the sflan vector layer, with circles drawn in blue and filled with a darker shade of blue. Lines 11-18 introduce a vlines block, which draws a portion of a layer called tiger. The where command in line 12 filters the vectors to draw only those that have a Census Feature Class Code (CFCC) attribute matching A1, which corresponds to limited-access highways. (The CFCC attribute is just a property of the particular data set used to draw the map.) These features are given the label "Freeways" and drawn in a red line 1 pixel wide, with a black highlight 1 pixel wide on either side. Major streets and ordinary streets are given corresponding treatment in lines 20-34. In lines 36-39, the contour lines from the sfcontour vector layer are drawn in brown. The legend and map information are placed in lines 41-49, and lines 51-54 display the contents of a text file called sf_tiger_hdr.txt across the header of the map, in 28-point type.

Interestingly, ps.map draws vector layers in the reverse order that they are listed in the map file.

ps.map is run in the following fashion:

GRASS:~ > ps.map input=sf_tiger.txt output=sf_tiger.ps

If you don't have a PostScript viewer, some free alternatives are available in [Hack #28] . If, for whatever reason, you prefer Encapsulated PostScript, ps.map will generate that instead, with the -e option. From there, you can use a tool like the epstopdf program that ships with GhostScript to convert PostScript or EPS to PDF, for example.

One drawback with ps.map is that it can only read one raster layer into a PostScript image, unless you're using three rasters to make RGB color composites. If you want to display the results of layering more than one raster image togetherfor example, the topographical height field as a transparent layer over a DRG street map [Hack #72] you have to first combine them into one raster layer, perhaps by using r.patch or r.composite.

As PostScript is a vector format, vector display is where it really shines, and you can layer as many vectors into your PostScript document as you like. ps.map has different vector options for importing shapes, lines, and points. However, the parameters are more or less the same for each vector option.

Obviously, with a tool as powerful as ps.map, we can only offer the outlines of its general functionality, and we strongly recommend perusing the documentation if you're interested in publishing your maps to the world this way.