Posted on Leave a comment

Why Georeference an Orienteering Map?

georeferencing image

What is georeferencing?

In simple terms for orienteering, georeferencing is aligning a map with the national map grid. On a georeferenced orienteering map, any point can be related to its location on the national grid and thus on the ground. The national projection is Map Grid of  Australia 94 (MGA94). The related datum is the Geodetic Datum of Australia 94 (GDA94) and is expressed in latitude and longitude.  GDA94 is consistent within 1 metre to the World Geodetic System 1984 which users of a GPS device may recognise as WGS84.GPS image

Why georeference O maps?

Having just mentioned GPS, the first response is that a georeferenced map enables import of GPS data or a GPX file with automatic correct placement on the map. On a non-georeferenced map you would be faced with moving via mouse, the GPS track and waypoints to the correct position – often a very tricky task. Data from a TruPulse laser rangefinder  can also be captured in the field (Professional edition) and appear instantly in the correct location on the map.

ortho photoA second response is that with a georeferenced map, ortho aerial photos such as provided by NearMap are automatically aligned with your map. Similarly for georeferenced data and georeferenced PDFs.  A huge time saving and likely a far more accurate result when transcribing detail from the photo.

A correctly georeferenced map minimises distortions in the map which can trouble elite orienteers. One map I have run on a number of times troubled even me. I had the opportunity to georeference it recently and found a section of it was so badly distorted that it also affected placement of features beyond that area.

When and how to georeferenceShape & DXF image

Ideally when the map is first created. Sources of data to create a georeferenced base map include Shape (Professional edition) and DXF files from state mapping and local government. Shape files generally offer the most useable data and are easiest to use. Open Street Map data may be useful for some types of map (Professional edition but see free service below).

Georeferencing existing maps is reasonably straightforward if you have access to map data that is already georeferenced. My preference is to georeference against GIS data. This is now free and easy to download in some states including Victoria. I usually use the roads data set from the Transport shape file as road junctions, as opposed to tracks, are fairly accurately positioned.

A scan of a topo map is also reasonable straightforward although a georeferenced digital data map is even easier.

Be prepared for distortions to show up especially if the map was not created by a professional mapper. OCAD will fairly successfully correct slight distortions. But if they are considerable and/or a section of the map is grossly distorted, then the Rubber Sheeting function in OCAD Standard and Professional is likely the only answer. OCAD website has videos on georeferencing and rubber sheeting.

Georeferencing service

While georeferencing is within the capabilities of most orienteering mappers, updating a library of maps may not be best use of your time. I offer a paid service to do the drudgery for you. See the Cartographic Services page for more information.

OSM import free serviceOpen street Map logo

Want to use OSM data but don’t have OCAD Professional edition? Read more...

Leave a Reply

Your email address will not be published. Required fields are marked *