2. Spatial Data Collection
How spatial data are collected?
Second hand - digitizing from maps.
To know how spatial data are collected, helps us to appreciate the possible level of errors or uncertainties involved in the data collection process.
In what forms are spatial data collected? How is spatial sampling done?
· Random collection
· Systematic sampling or complete coverage
· Other hybrids of the first two
One needs to determine the density of sampling, obviously, the denser one collects data, the more likely one would represent the reality.
The density of sampling is a function of a number of factors,
(1) the complexity of the phenomena,
(2) the capability of the measuring tools
(3) the available accuracy requirement
(4) economic considerations
At most of the time, we tend to use second-hand spatial data, i.e., currently available data and they are often in map forms.
How are maps made?
For thematic maps,
(1) Manually · Base map preparation
· Thematic data transfer (from survey, aerial photographs remote sensing images) on to the base map
· Interpolation or extrapolation may be needed
· Classification, generalization, symbolization and decoration
· Layer separation and printing
(2) Computer-assisted · Base map preparation
· Geometric transformation (include interpolation)
· Data conversion, classification, generalization
· Legend design and decoration
For base maps,
· Select proper map projection
- preserving area
- preserving length
- preserving direction
· Data transfer
· Interpolation or extrapolation
· Generalization, symbolization and decoration
Reference: Robinson, Elements of Cartography
What is a thematic map?
What is a base map?
What is a reference map?
What is a topographic map?