Georeferencing Existing Orienteering Maps
What is Georeferencing?
In simple terms, when you take any ordinary image (or map) and give it real-world coordinates, you are georeferencing it.
More info can be found here:
◪ British Orienteering Map Group document on georeferencing.
◪ OCAD wiki: Create a new map.
To use Routegadget effectively
Auto-alignment of other georeferenced data such as ortho aerial photos and Lidar.
When to Georeference maps
Generally, it makes sense to georeference a map when starting a new mapping project but sometimes it’s necessary to georeference an existing map that has been drawn to OCAD’s paper co-ordinate grid.
This is a relatively simple task and there are a number of different ways to do it.
I will go through the simplest way to do this using OCAD 2020.
Step 1 - Seperate map and layout content
In order to georeference the map we will need to rotate, scale and move the map into position. Therefore, it makes sense to separate the layout content from the map content so the layout isn't affected by the transformations.
The easiest way to separate is to create two copies of the original file, delete all the map content from one and all the layout content from the other.
Use Map > Optimise / Repair on both files to shrink and remove deleted object data.
Keep the North-Lines with the layout content file and delete from the map content file.
From Step 2 - 6 we will be working with the seperated map content file.
Step 2 - Find out the map scale
Open up the map file in OCAD and go to: Map > Set Scale and Coordinate System…
Take a note of the scale shown and make sure the Coordinates for the map are set to paper coordinates. Click OK and then close the map file for now.
Step 3 - Use New Map Wizard to auto-generate a georeferenced map of the existing map area
Select the Great Britain data source and one of the symbol sets to match the map you are georeferencing. ISOM 2017 for forest, ISSPrOM 2019 for sprint. **Matching the symbol sets is not essential but it might make it easier to line things up later if the symbols on both maps are close in size.**
Change the Map scale to the same as the existing map.
Find the area covered by the existing map. Click Create >
**No need to check 'Rotate map to magnetic North' at this stage**
OCAD will then download the Open Street Map data and generate you a fully georeferenced base map.
Save this new map somewhere you will remember and close the map.
Step 4 - Load auto-generated basemap as a background map
Open up the existing map in OCAD
Open the auto-generated basemap as a background map: Background Map > Open…
In the 'Map offset' box select 'New offset' and make sure the details match the details above in the Background map information box.
Step 5 - Align existing map to the georeferenced background map
In this step we will use Affine Transform to align the existing map to the background map.
Firstly, to make it easier to see what’s going on under the map when you align you can make the existing map transparent by going to View > Draft Mode
Click a point on the map (Road/stream junctions are useful reference points.)
Click a point on the background map.
Repeat the process again so you have 2 points marked. This will be enough to roughly align the map.
Press Enter on the keyboard to align the map to the background map.
There are instructions displayed at the bottom left of the window to help.
Marking 2 points will be enough to roughly align the map but use more if you need to better align the map.
OCAD will fairly successfully correct slight distortions with the affine tool. But sometimes, for old maps, the whole map may be locally distorted. In this case, it may not be worth trying to match the old map to the georeferenced background map. Instead, I would suggest this might be a good time to start a new map of the area from scratch.
OCAD offers a solution to this problem if the distortions aren’t too drastic in the form of a rubber sheeting tool. More information about this method can be found here: https://www.ocad.com/wiki/ocad/en/index.php?title=Map_Transform#Rubbersheeting
Once you are happy with the alignment you can remove the background map: Background Map > Manage...
Step 6 - Rotate map to Magnetic North
The next step is to rotate the map from OS Grid North to the correct Magnetic North.
This is a very simple process using OCAD 2020:
Step 7 - Re-align the layout content with the newly georeferenced map
The final step is to take a note of the Coordinate figures and MN setting from the map : Map > Set Scale and Coordinate System… and go into the Layout file and set that to same values as the map file.
You can then open the map file as a background map in the layout file and then move the layout around to work with the transformed map.
You may need to draw new North Lines depending on how much the map has been transformed.
Firstly, delete the old North lines.
Then, to generate new lines: Map > Create Map Grid...
Tick Create vertical grid lines
Set the Distance to the required amount stated in the relevant specification. (300m Forest, 120m Sprint)
Select the North Line symbol