September 6, 2011 § Leave a comment
It should always be remembered that maps must be viewed as historic texts. They are not unbiased and are culturally and politically laden.
(As evidenced in Grace Karskens: On the Rocks commentary on engraved but non-existent buildings, James Meehan’s depiction of a planned Port Phillip (never completed) in his 1807 Plan of the Town of Sydney, and Thomas Mitchell’s 1835 Saint Phillip Parish map in response to the Governor’s request for division of land. Though these maps are close spatially and in time, they can not be compared with one another as they represent different things.) (Modern data collection can be done with conventional databases, digital vector data sources, digital survey methods, map digitising (historical maps), aerial photographs, satellite images and geophysical methods.)
When analysing maps (which are historical documents) they must be interpreted and analysed regarding the: Purpose, Audience (and resulting biases), along with the Intention of the creator, their skill and accuracy.
The act of integrating historical maps with GIS, georegestration, can reveal discrepancies of two general types:
1. Observable patterns of physical damage or map distortion.
2. Extrinsically identified errors in the depiction of works that might have been proposed, planned or simply Fanciful.
(Such discrepancies can be observed in the 1903 Surveyor General map, especially at Bridge Street, where the overlay of maps reveals drafting errors. The 1843 Baker Map, being a commercial map, attempts to maintain longevity by representing future plans (even if incomplete or not begun).
To derive data from historic maps, we first analyse it, and then integrate it with GIS. Existing vector data (e.g. Australian Land Ownership) helps act as anchors, being reliable documented sources (accuracy, pedigree, etc.). There is major research effort done into time stamping data, but not spatial data acquisition. Map data tends to be accumulative, able to be timestamped and later subtracted.
Geo-referencing involves some technical terms.
- Geo-referencing link tables store the image location as X and Y Source. Co-ordinates (according to MGA) are stored as X and Y Map.
- ESRI Shape Files are a common open standard, but consist of multiple files (.shp , .shx , .dbf , .sbn and .sbx , .prj , .xml (modern) ).
- KML = Keyhole Markup Language
Now an officially accepted standard with many online tools.
- KMZ = Compressed KML, and can be opened in Google Earth.
- Wavelet compression ‘loads more detailed images as you zoom in’.