Georeferencing Orienteering Maps in OCAD

Georeferencing assigns real-world coordinates to a map or image, enabling integration with tools like GPS and other geospatial data. This guide explains how to georeference an orienteering map in OCAD Orienteering Edition, ideal for new mapping projects or aligning existing maps drawn to OCAD’s paper coordinate grid.

Why Georeference?

• Enables GPS tracking for accurate navigation.
• Supports tools like Routegadget for route analysis.
• Allows auto-alignment with georeferenced data, such as ortho aerial photos or LiDAR.

For more details, see:

• British Orienteering Map Group on Georeferencing
• OCAD Wiki: Create a New Map

Step-by-Step Guide

Step 1: Separate Map and Layout Content

To georeference without affecting layout elements (e.g., title, legend), separate map and layout content:

1. Create two copies of the original OCAD file.
2. In one copy, delete all map content (keep layout elements, including North Lines).
3. In the other copy, delete all layout content (remove North Lines).
4. Run Map > Optimise/Repair on both files to remove deleted object data.

Note: Work with the map content file for Steps 2–6.

Step 2: Check Map Scale

1. Open the map content file in OCAD.
2. Go to Map > Set Scale and Coordinate System.
3. Note the map scale and ensure coordinates are set to Paper coordinates.
4. Click OK and close the file.

Step 3: Create a Georeferenced Base Map

1. Go to File > New Map Wizard.
2. Select Great Britain as the data source and choose a symbol set matching your map (e.g., ISOM 2017 for forest, ISSPrOM 2019 for sprint). Matching symbol sets helps with alignment but isn’t mandatory.
3. Set the map scale to match the existing map’s scale.
4. Locate the area covered by your existing map and click Create. (Do not check Rotate map to magnetic North.)
5. OCAD will download OpenStreetMap data and generate a georeferenced base map.
6. Save this base map in a memorable location and close it.

Step 4: Load Base Map as Background

1. Open the original map content file in OCAD.
2. Go to Background Map > Open and select the georeferenced base map.
3. In the Map offset box, choose New offset and verify the details match the Background Map Information box.
4. Click OK.

Step 5: Align Map to Georeferenced Base Map

Use the Affine Transform tool to align the existing map with the base map:

1. To see the background map clearly, enable View > Draft Mode to make the existing map transparent.
2. Go to Map > Transform > Affine.
3. Click a point on the existing map (e.g., a road or stream junction).
4. Click the corresponding point on the background map.
5. Repeat for a second point (two points are usually sufficient for rough alignment; add more for precision).
6. Press Enter to align the map.
7. If satisfied, remove the background map via Background Map > Manage.

Notes:

• Follow the instructions displayed in the bottom-left corner of OCAD.
• Affine Transform corrects minor distortions. For heavily distorted old maps, consider creating a new map or using OCAD’s Rubber Sheeting tool for localised adjustments. See OCAD Wiki: Map Transform.
• If alignment is poor, it may be better to start a new map from scratch.

Step 6: Rotate to Magnetic North

1. Go to Map > Transform > Rotate Map to Magnetic North.
2. OCAD will adjust the map to align with magnetic north.

Step 7: Re-align Layout with Georeferenced Map

1. Open the map content file and note the coordinate system and magnetic north settings via Map > Set Scale and Coordinate System.
2. Open the layout content file and apply the same coordinate system and magnetic north settings.
3. Load the georeferenced map content file as a background map in the layout file via Background Map > Open.
4. Adjust the layout elements to align with the transformed map.

Step 8: Update North Lines

If the map transformation significantly alters orientation, update the North Lines:

1. In the layout file, delete the old North Lines.
2. Go to Map > Create Map Grid.
3. Check Create vertical grid lines.
4. Set the distance per the relevant specification (e.g., 300m for forest, 120m for sprint).
5. Select the North Line symbol and click OK.

Note on File Types: Background maps download as PNG files with corresponding .pgw files containing georeferencing data. Keep these files together in the same folder to ensure correct loading in OCAD.

Tip: Organise your project folder to easily locate map and layout files for future use.