2. Spatial Data Collection

• How spatial data are collected?

First hand  

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

Printing

For base maps,

Select proper map projection

- preserving area

- preserving length

- preserving direction

Data transfer

Interpolation or extrapolation

Generalization, symbolization and decoration

Printing

 

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?