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Georeferencing Historic Maps

Intro

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

Examples

Georeferencing by Coordinates

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.

Resources

Coordinate Transformation

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

Geographic datum (short) Map 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.

 AGD Technical Manual - p52

 AGD Technical Manual - p53

 AGD Technical Manual - p54

Coordinate Conversion

PROJ is a useful program here.

'Clarke Coordinate' have the following parameters

Parameter Value Units
ellipsoid Clarke 1858
a 20,926,348 british feet
a 6,378,339.78 metres
b 20,855,233 british feet
b 6,356,663.92 metres
1/f 294.26
projection transverse mercator
false easting 400,000 yards
false easting 365,759.36 metres
false northing 800,000 yards
false northing 731,518.73 metres
zones 1 to 8
zone width 5 degrees
zone 1 central meridian 116E degrees
zone latitude origin 34S degrees
scale factor 1

Note:

  • The Clarke 1858 ellipsoid parameters used here are specified in british feet, with a conversion factor of 0.30479947 metres per british feet
  • Differing values in metres are quoted online for the ellipsoid, however, this appears to be due to different conversion factors:
    • Tasmania apparently used the Clarke foot which equalled 0.3047972654 meters
    • Others appear to use today's standard conversion of 0.3048
  • Need to confirm what the approporiate conversion factor for the yards used in the false easting and northing should be (3*british foot or 0.9144)
    • PROJ allows you to specify your output units, however, it doesn't allow you to define them. It defines yd=0.9144 so I've supplied the false e/n using in metres using the same conversion factor. Testing shows that the conversion is approximately correct near the true origin, however, I think it will grow larger the further away. Using the british feet conversion factor for both false e/n and output should correct this.

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=[ZONE CM] +k_0=1 +a=6378339.78 +b=6356663.92 +units=yd +x_0=365760 +y_0=731520

As an example, converting the lat,long (S 33.75, E 151.5) to E,N for zone 8:

echo 151.5 -33.75 | proj +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

Returns 450,666.69 830202.59

georef.1539105648.txt.gz · Last modified: 2018/10/10 04:20 by allchin09

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