Scale Resolution and Accuracy in GIS Because GIS

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/gisscale. html

NHD at two spatial scales 1: 100, 000 scale 1: 24, 000 scale

Streams and Map Scale 1: 100, 000 scale 1: 24, 000 scale 20 times no. of lines 3 times total length of lines San Marcos Basin Flow Lines 1: 100, 000 scale – 557 lines, Total length 1890 km San Marcos Basin Flow Lines 1: 24, 000 scale – 11, 338 lines, Total length 5559 km

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

From http: //www. colorado. edu/geography/gcraft/notes/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_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 1/30 th of an inch for maps at a scale of 1: 20, 000 or larger, and 1/50 th 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_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