Custom map images can be added to the app using the following format.

Projection: NZTM2000

Tile size: 1024 by 1024 pixels

File format: jpeg

Filename format: mzzxxxxyyyy.jpg where zz is the zoom index, xxxx is the easting index from the reference easting, and yyyy is the northing index from the reference northing. Easting and northing indexes are zero based and relative to the western and northern most corner (top left) of the image.

Reference easting: 1560000 (North), 1080000 (South)

Reference northing: 6200000 (North), 5550000 (South)

Scale: 0.47244 pixels per meter for zz=10, 0.23622 pixels per meter for zz=09, 0.11811 pixels per meter for zz=08, 0.05905 pixels per meter for zz=07, etc

To use the custom images, save them to a directory on your phone and then select this directory from the custom map selector on the settings page in the app. The map layer button will now cycle between the topographic map, the aerial images, and your custom images (with topographic images filling in the gaps). The current version of the app displays images at zoom levels 03,05,07,09,10. Other zoom levels are ignored.

Example calculation: North Island tile m0700010001.jpg (zoom level 07, easting index 0001, northing index 0001). The scale for zoom level 07 is 0.05905 pixels per meter, so a 1024 by 1024 pixel image will span 17,340 m (1024/0.05905). The top left coordinate for the image will be 1560000+17340*0001 = 1577340 east, and 6200000 – 17340*0001 = 6182660 north. The bottom left coordinate will be offset by a further 17340 m (1594680 east, 6165320 north). Using the LINZ topo250 series, the image is a follows.

Example application: For a touch of old school navigation courtesy of James Cook, download and unzip these files to your phone (North Island James Cook Map). Select the folder under Custom Maps in the app settings, and then cycle the topo/satellite map button until you see the ‘C’.