Generalizing arcs

National/provincial maps

1. ensure that the coverage is in a projection like Lambert, where North-South and East-West distances are roughly equivalent. (The coverage must not be in decimal degrees.)

    ARC:  PROJECT COVER covername covername2 dd2lam.PRG
   

2. Determine the number of polygons, and ensure that no label errors exist.

    ARC:  DESCRIBE covername2
   

3. Generalize the coverage with a weed tolerance of 0.05mm at the scale of the original map.

    ARC:  TOLERANCE covername2 snap 0.001
    ARC:  TOLERANCE covername2 nodesnap 0.001
    ARC:  GENERALIZE covername2 covername3 weedtolerance
   

   where weedtolerance =
   • 1,500 m @ 1:30M
   • 375 m @ 1:7.5M
   • 300 m @ 1:6M
   • 50 m @ 1:1M
4. Rebuild the topology.

    ARC:  BUILD covername3
   

5. Ensure that the number of polygons has not changed, and that no label errors exist.

    ARC:  DESCRIBE covername3 
   

   If the number of polygons has changed, and label errors exist, divide the weedtolerance by 2 and start over at the generalization step.
6. Project the cover back to decimal degrees and rebuild the topology (if necessary).

    ARC:  PROJECT COVER covername3 covername4 lam2dd.PRG
    ARC:  BUILD covername4
   

Detailed maps

1. ensure that the coverage has been projected.

    ARC:  PROJECT COVER covername covername2 ddtolam.PRG
   

2. Determine the number of polygons, and ensure that no label errors exist.

    ARC:  DESCRIBE covername2
   

3. Generalize the coverage with a tolerance factor of .05mm at the scale of the original map.

    ARC:  TOLERANCE covername2 snap 0.001
    ARC:  TOLERANCE covername2 nodesnap 0.001
    ARC:  GENERALIZE covername2 covername3 weedtolerance
   

   where weedtolerance =
   • 25 m @ 1:500,000
   • 6 m @ 1:126,720
   • 5 m @ 1:100,000
   • 3 m @ 1:63,360
   • 2 m @ 1:50,000
   • 1 m @ 1:20,000
4. Rebuild the topology

    ARC:  BUILD covername3
   

5. Ensure that the number of polygons has not changed, and that no label errors exist.

    ARC:  DESCRIBE covername3 
   

   If the number of polygons has changed, and label errors exist, divide the weedtolerance by 2 and start over at the generalization step.

References

Monette, B., and B. K. MacDonald. 1992. Standards of accuracy for digital soil resource data stored in the National Soil DataBase (NSDB). GIS for the 1990's. Ottawa, March 24-26 1992. pp. 282-288. Contact: Bryan Monette