Scale, Resolution and Accuracy in GIS

• Because GIS data is stored in a very different way than paper map data, the relationships between map scale, data accuracy, resolution, and density are different between GIS and paper maps.

• A map in a GIS can be shrunk or enlarged at will on the screen or on paper. You can zoom in until the screen displays a square meter or less, or zoom out until the screen displays the entire earth. This means that geographic data in a GIS doesn't really have a 'map scale'.

From: http://srmwww.gov.bc.ca/gis/gisscale.html

NHD at two spatial scales

1:100,000 scale

1:24,000 scale

Streams and Map Scale

San Marcos Basin Flow Lines1:100,000 scale – 557 lines, Total length 1890 km

San Marcos Basin Flow Lines1:24,000 scale – 11,338 lines, Total length 5559 km

1:100,000 scale 1:24,000 scale

20 times no. of lines3 times total length of lines

Length of the San Marcos River

1:100,000 scale – 39 records, total length = 130.0 km

1:24,000 scale – 112 records, total length = 138.1 km

6% longer and 2.9 times no. of lines for 1:24,000 scale

San Marcos River

From http://www.colorado.edu/geography/gcraft/notes/error/error_f.html

Scale, Resolution and Accuracy in GIS

• Accuracy is the degree to which information on a map or in a digital database matches true or accepted values.

• Precision refers to the level of measurement and exactness of description in a GIS database.

From http://www.colorado.edu/geography/gcraft/notes/error/error_f.html

Relating scale and accuracy • United States Geological Survey mapping standards: "requirements

for meeting horizontal accuracy as 90 per cent of all measurable points must be within 1/30th of an inch for maps at a scale of 1:20,000 or larger, and 1/50th of an inch for maps at scales smaller than 1:20,000."

• 1:1,200 ± 3.33 feet • 1:2,400 ± 6.67 feet • 1:4,800 ± 13.33 feet • 1:10,000 ± 27.78 feet • 1:12,000 ± 33.33 feet • 1:24,000 ± 40.00 feet • 1:63,360 ± 105.60 feet • 1:100,000 ± 166.67 feet

From http://www.colorado.edu/geography/gcraft/notes/error/error_f.html

http://geography.usgs.gov/standards/

http://rockyweb.cr.usgs.gov/nmpstds/nhdstds.html

STREAM/RIVER - A body of flowing water.

Horizontal data are confidently positioned within 0.02", at map scale, of the true ground position. Vertical data are confidently positioned within one-half contour interval of the true ground position

0.02" x 100,000 = 2000" = 167 ft = 50 m

0.02" x 24,000 = 480" = 40 ft = 12 m