Pima County Department of Transportation

Geographic Information Services Division

Developing GIS data for the Web

Generating and viewing CGM (vector) files

Reasons for using CGM files

Unlike raster GIF files, Computer Graphics Metafile (CGM) files store vector information. Vector files allow viewing with pan, zoom and no loss of detail. The biggest disadvantage is that CGM files cannot be viewed with native browsers such as Netscape. Another disadvantage is that CGM files are much larger than GIF files, perhaps 10 times larger.

Since text in a CGM file is stored in vector form, text on maps makes CGM files much bigger. Your CGM files will be much smaller if you remove text prose (paragraphs, etc). On the web, this text is more appropriate in an HTML file rather than part of a map. It's clearer and almost inconsequential in size as compared to text in a CGM file.

Viewing CGM files

CGM files can be viewed directly on the web with a web browser plug-in. We have tried two plugins described below with mixed results. The technology is promising. Results are better than it might appear here. It's also possible that there are errors in the ARC/INFO generated CGM files that are causing these problems. It may also be that GIS plots result in bigger and more dense files than these CGM viewers are used to. They certainly can have more information than a typical mechanical assembly drawing.

One such plugin is InterCAP Graphic Systems' InLine for Netscape. You can download InterCAP InLine from InterCAP Graphic Systems (Pick ActiveCGM) or through another InterCAP site. InterCAP InLine requires Windows 95 or Windows NT. Sometimes, InterCAP Inline has had problems for us:

Fails to display anything for some CGM files.
Polygon fill sometimes gets confused and fills "outside the lines".

Figleaf from Carberry Technology is a browser plugin that serves the same general purpose. As with InterCAP Inline, Figleaf has had a few problems:

Portions of the image may be cut off.
Portions of text may not display.
Fails to display anything in zoomed areas that have some missing text.
Additional portions may not be displayed after zooming and unzooming.
Colors are sometimes incorrect, but have been seen to correct themselves after zooming and unzooming.

Another CGM plug-in viewer is Metaweb from Henderson Software. We haven't tried it yet.

Generating CGM files from ARC/INFO

There are at least two ways to generate CGM files with ARC/INFO.

Use the CGM command to convert a GRA file to a CGM file.
Generate the CGM file directly with ArcPlot.

The two techniques should be equivalent. However, they produce CGM files of slightly different sizes and the CGM viewers may react slightly differently depending on the technique used. This suggests the possibility that there could be errors in the CGM files. We have had better and more consistent results using the CGM command.

If you already have an ESRI graphics (.GRA) file, then it is easy to convert it to CGM format with the following ARC command:

   cgm infile.gra outfile.cgm # binary

ArcPlot can generate CGM files directly with the these changes to the typical plot AML:

    .
    .
   display 1040 6                (The "6" specifies binary CGM file format.)
   filename.cgm                  (Add "cgm" for the generated file name.)
    .
    .
   /* draw filename.gra 9999 3   (Comment out the draw statement.  You
                                  can't display the CGM file in Arc -
                                  that I know of....)

Or, make the original aml produce both GRA and CGM files. For example:

  
   &echo &brief
   &sv covpath = /pace2/washes/   /* Or whatever....
   arcplot

   &do outtype &list gra cgm /* Loop once with gra
                             /* and again with cgm.

   &select %outtype%
   &when gra
     &do
       display 1040    /* Defaults to type 1, gra 
       washes          /* Defaults to washes.gra  
     &end
   &when cgm
     &do
       display 1040 6  /* "6" specfied Binary CGM 
       washes.cgm      /* Note the cgm filename extension. 
     &end
   &end /* end select */ 
      .
      .
   /* The "meat" of your ArcPlot drawing aml goes here. 
      .
   /* You may need to rearrange the plot order of individual
   /* plot elements to be appropriate for screen viewing
   /* which does not blend output like most plotters.
   /* For instance, lines in a poly must be drawn after
   /* poly shading.
      .
      .
   &end   /* End of loop through output types: gra and cgm. 
   &echo &off
   quit
   draw washes.gra 9999 3   /* Preview the gra file on the screen.