Process of digitization involves creation of vector layers using digitizing tools. It is a method of converting raster data into vector data.
In this process a satellite image covering area of interest or raster map containing desired features is used as background and features are traced over it with the help of digitizing tools available in the GIS software. Separate vector layers are created for different features for example- roads, rivers, railways, water bodies, land use, wells, etc.
Things to remember
1. First georegister (or georeference) the satellite image (if it is raw) or raster map then start digitization over it. Vector layers created in this way will automatically pick projection from raster layer in background. If you digitize features over raw images (i.e. without projection) then you will need to georeference each and every vector layer which will consume a lot of time and resources.
2. Define snapping tolerance before starting digitization so that arcs will be properly connected and dangles will not be created.
3. If you are digitizing polygons, do not re-digitize common boundary between them. Use specialized tool provided in the software- using which you will need only to digitize boundary which is not shared by adjacent polygons, the common boundary it will take automatically.
4. It is always better to attach attributes simultaneously while digitizing, it will save lot of time and chances of attaching wrong attributes will be negligible.
5. Tolerances should be defined very carefully large tolerance values may remove legitimate lines and polygons while very small values may not remove all topological errors (dangles and slivers).
6. Digitization requires a lot of patience and hard work. If done in hurry it may degrade the quality of desired output. Use of improperly digitized vector layers in subsequent analysis and modeling may lead to erroneous results.
Digitization Errors
Dangles
Dangles are topological errors where an arc or a line does not end at the point where it should. These are created due to improper digitization. Dangles are of two types- overshoots and undershoots.
1. Overshoots
When an arc or a line does not end at its termination point on another arc and goes beyond it is called as overshoot.
2. Undershoots
When an arc or a line finishes before connecting to another arc on desired location it is called as undershoot.
Spurious Polygons
Spurious polygons or slivers are often created during overlay of two or more polygon layers. Slivers are small polygons which results due to overlay operations of polygons whose edges do not match.
Avoiding/removing Dangles and spurious polygons
Dangles can be avoided it proper snapping tolerance is defines before starting digitization. Even then if dangles are created then these can be removed during ‘cleaning’ of vector layers- at that time also desired tolerance should be set and cleaning is done which automatically removes dangles.
Spurious polygons can be removed during cleaning operation by setting proper tolerance values.
Please see INDEX for complete list of topics.
