This project aims to georeference historic maps which are of interest to bushwalkers in NSW, and make them available in a format which allows for easy viewing.

General parts involved are:

• Obtain digitised map through either
  • Obtaining physical map and digitising using scanner or camera
  • Downloading / acquiring a map that has been digitised by someone else (eg from the NLA via Trove)
• Georeference map
  • By features
    • Manually, by picking common points in both old map and one in the desired coordinate system
    • Automated, through some algorithm that can identify common points and perform the process
  • By coordinates
    • Making use of the coordinates listed on the map (if available) and using a known coordinate transformation
• Display online
  • Ideally would be a large tiled map for maps that are of a series, such as the LPI S1 topo
  • A multi-layered version, where available maps are based on currently viewed bounds, see USGS Historic Topographic Map Explorer

Here, twelve 1:63,360 scale military topographic maps were georeferenced and overlayed on the NSW LPI basemap. These digitised maps were sourced as low-quality JPGs (large TIFs also available) from the NLA (National Library of Australia) via Trove.

The maps were then georeferenced using the lat,long coordinates of corners of each map. These maps were generally published to cover 0.5 deg of long by 0.25 deg of lat, resulting in a regular grid of maps. Finally, they were clipped to cover only their mapped extent, removing the boards and information contained within.

No attempt was made to correct for the difference between the ANG and current GDA datums, so although the maps generally align with each other well, there can be discrepancies of 100s metres between the georeferenced maps and the LPI basemap.

Further work should be done in exploring variations of this process, particularly if a datum transformation can be applied.

When using old maps which have a CRS (coordinate reference system) significantly different from that which we use today, it can be useful to be able to transform coordinates from one CRS to another.

A brief overview of 'grids & datums' history for Australia can be found in this 2003 publication

Transformations listed before are quite well known, and many GIS packages can be used:

• AGD66 ↔ AGD84
• AGD66 ↔ GDA94
• GDA94 ↔ GDA2020
• GDA94 ↔ WGS84

Note that these refer to geographic datums, rather than map grids of a projection

Some details for pre AGD66 transformations can be found in the AGD technical manual (pp 52-54) which have been included below for reference.

PROJ is a useful program here.

'Clarke Coordinate' have the following parameters

Using these details, the below PROJ code will convert lat,long to E,N for a given zone.

+proj=tmerc +south +lat_0=-34 +lon_0=151 +k_0=1 +a=6378339.78 +b=6356663.92 +units=yd +x_0=365760 +y_0=731520