Map Scale and GIS GEO426/526 Principles of GIS

Scale is the ratio of map distance to ground distance (e.g., one map inch represents one ground mile) – Word/verbal statement – Graphic – Representative fraction Large and small scale Generalization in GIS = different scales of data Scale

Is GIS “scaleless”? Can easily zoom to any scale, so scale doesn’t matter? Original scale 1:5 million Original scale 1:25 million

Is GIS “scaleless”? Can easily zoom to any scale, so scale doesn’t matter? 1:500,000,000 1:6,000,000

Raster example USGS topographic map

Is GIS “scaleless”? Evident that original data scale (reference scale) is preferred for appearance’s sake From ArcCatalog: – Supplementary Information Largest scale when displaying the data: 1:50,000,000. Rule of thumb: – Don’t zoom in or out beyond two times original data scale 1:6,000,000

Scale and resolution Vector data resolution is the sampling distance of the stored x-y values. Finer resolution needed for large scale maps Coarser resolution needed for small scale maps Raster data resolution = cell size 1:5M scale source1:25M scale source Display scale approximately 1:500,000

Is GIS “scaleless”? Not so evident, but true that accuracy tied to original data scale USGS Topographic map accuracy: – “This data complies with United States National Map Accuracy Standards for 1:24,000 scale maps. According to this standard, not more than 10 percent of the points tested are to be in error by more than 1/50 inch (40 feet) measured on the publication scale of a USGS 7.5 minute topographic quadrangle map.” All data has similar limitations whether stated or not.

Finding source scale Usually documented in the metadata – Scale of original paper map source – Scale or precision at which data were gathered

Scale and your semester project Scale at which processes operate Segregation? Breastfeeding decisions? Deforestation? Urban climate effects? Incarceration rates?