The simplest way to vectorize a raster layer is to digitize vector objects on a map. Manual digitizers are least expensive and the most commong means of capturing vector objects from paper maps.
A geographer can convert a paper map into digital form by using a digitizing board or by digitizing at the computer using "heads-up" technology. The process is called "registering." A map is placed on the digitizer board. The corners are selected by a mouse click and their coordinates are entered using the keyboard. Then the points on the map are digitized using a mouse. As the user traces each of the map's contours, a series of (x,y) coordinates that define that contour are sent from the tablet to the computer. The user always sees the map.
Once points and lines have been digitized, there must be quality control. There is always the possibility of human error when digitizing. Checking for errors is done by redigitizng, printing or plotting the map for overlay onto the original map, or by mathematical calculations.
Italian company ITC makes free GIS software. They've set some basic rules for registering raster maps. These rules include: 1. If two lines cross, you need to place a node (point) where the two lines intersect. 2. Lines that share a node (point) need to actually touch that node exactly. 3. A person scanning an area's common boundaries should only scan those boundaries once. No redundancy. 4. When scanning a polygon (square, etc.) make sure your last point connects to the first point, closing the polygon. You don't want to leave it open. 5. Store lines and areas in separate maps.
The Library of Congress has a National Digital Library Program. Historical maps can be accessed more readily once they are ditigitized. However, most maps are oversize which requires special handling and special scanning equipment. The Center for Geographic Information was formed in 1995 to support the Library of Congress’s Geography and Map Division in its transition to the digital world.
Many companies manufacture digital scanning tablets. These companies include Calcomp, Summagraphics, Altek, GTCO, Numonics.
- <Longley, Paul, et. al. Geographic Information Systems and Science. 2005.>
- <Neteler, Markus, et al., Open Source GIS: A Grass GIS Approach. 2004. Springer US Publishing>
|Authors||Laurie H., Rob Sanders|
|Tags||vector, digitizing, maps, coordinates|