Getting the Map into the Computer Getting Started with Geographic Information Systems Chapter 4.
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Getting the Map into the Computer Getting Started with Geographic Information Systems Chapter 4
Data requirements and sources Most economic uses of geographic data require: u cartographic data u publicly available demographic and economic data u privately available data u proprietary data
GIS cartographic data can be u Purchased. u Found from existing sources in digital form. u Captured from analog maps by GEOCODING.
GIS attribute data can be u Purchased u Found digitally or on maps u Proprietary, in digital or non-spatial form u GEOCODING u ADDRESS MATCHING
Address matching u Much customer, vendor, or facilities data are available by street address, or at least ZIP Code. u The spatial data handling functions of most GISs can assist with address matching into a spatial database, given required data. u There are always problems.
Your Spatial Data “Rights” u US Federal u FOIA u COFUR u State u Local u Protection for national security, proprietary info, privacy u Attributes vs. map data
GEOCODING u Geocoding is the conversion of spatial information into digital form. u Geocoding involves capturing the map, and sometimes also capturing the attributes.
Geocoding methods for maps u Digitizing u Scanning u Field Data Collection
GEOCODING LEAVES A “STAMP” ON DATA u The method of geocoding can influence the structure and error associated with the spatial information which results. u Example: scanning (raster), digitizing (vector).
Digitizing u Captures map data by tracing lines from a map by hand u Uses a cursor and an electronically-sensitive tablet u Result is a string of points with (x, y) values
Scanning u Places a map on a glass plate, and passes a light beam over it u Measures the reflected light intensity u Result is a grid of pixels u Image size and resolution are important u Features can “drop out”
The Role of Error u Enforcement for map data is usually by using topology. u Map and attribute data errors are the data producer's responsibility, but the GIS user must understand error. u Accuracy and precision of map and attribute data in a GIS affect all other operations, especially when maps are compared across scales.
Geocoding other records u A common operation u Based on some geographic attribute of records that our software can relate to geographic coordinates: u street address (TIGER or private match data) u ZIP codes or Census tracts (allocate to centroids)