Compilation and Digitizing

Compilation: Assembling and fitting together of the diverse geographical data to be included on the map. Involves generalization; Also involves map error (see 26)

What information - depends on application (theme) and scale
Base map: provides basic spatial reference (but one person's base may include features not considered "basic" by others)
Thematic Overlay: the objects of interest must be compiled too. Distinction?

Large Scale maps (detailed) [ remember that 1:10000 is "larger than" 1:1000000...]
often compiled photogrammetrically or through field measurements
attributes from direct sources (tabular lists of parcels, streets, etc.)
Small scale maps Dent calls them thematic base maps - derivative maps
based on reductions of large scale maps
Sources: see Dent Appendix C &D for Federal, but even more at local level
Key concepts:
Registration of different sources - connected ultimately to a coordinate system
Working by separation (based on theme) - remember the Sherman maps
Logical consistency

Methods of compilation in manual cartography

Projections: graphical construction and recompilation (e.g. by squares)
at same scale: one-to-one (copying), simplification/generalization/selection
reduction: reduce source material (optically) and redraft linework
Compilation involves weaving together conflicting evidence, the cartographer adjusts the sources to make them match.
Parcel descriptions do not always describe exactly the same boundaries, but the social purpose is clear that there is meant to be only one owner.
Adjacent photographs (or map sheets) may not match; line must adjust somehow.
Lines are used as a graphic expression of a logical boundary, they have to go somewhere - but it does not mean that the transition is that sharp.
Some lines trace exact boundaries (eg. political subdivisions), but lines on a soil or vegetation map must be interpreted as transition zones (topology still applies).

Digital compilation through use of digitizers

Digitizer equipment makes map measurements under software control (at scale).
Projections, scale reductions even generalization are numerically automated.
Digital databases: mostly derivative now, due to backlog of existing maps, will replace new compilation eventually. Detailed sources exist.
Same problems and rules, except that the computer can perform many of the tests of logical consistency automatically (using the topological data structure)

• Do lines cross only at nodes? (intersection check)
• Are any lines entered twice?
• Are all areas completely described (closed)?
• Is the total area assigned to one and only one polygon?
• Are there any undershoots or overshoots?
• Are any polygons too small or lines too short or too close? (technical limits)