Understanding Georeferencing: A Practical Guide
Georeferencing, in simple terms, involves assigning real-world coordinates to an ordinary image or map. Here's why it matters and how to do it:
Why Georeference?
- GPS Tracking: Enables precise location tracking.
- Routegadget: Enhances effectiveness in using Routegadget.
- Auto-alignment: Aligns other georeferenced data like ortho aerial photos and Lidar automatically.
When to Georeference Maps
Typically, it's done at the start of a new mapping project. However, sometimes it's necessary for existing maps drawn to OCAD’s paper coordinate grid.
How to Georeference
Follow these steps using OCAD Orienteering edition:
Step 1 - Separate Map and Layout Content
Before georeferencing, separate map and layout content to prevent layout alterations during transformations:
- Create two copies of the original file.
- Delete map content from one and layout content from the other.
- Optimize both files and retain North-Lines with the layout content file.
Step 2 - Find the Map Scale
- Open the map file in OCAD and note the scale.
- Ensure map coordinates are set to paper coordinates.
Step 3 - Auto-generate a Georeferenced Map
- Use the New Map Wizard.
- Select the appropriate data source and symbol sets.
- Match the map scale to the existing map.
- Define the area covered by the existing map.
- OCAD will generate a fully georeferenced base map.
Step 4 - Load Auto-generated Basemap
- Open the existing map in OCAD.
- Open the auto-generated basemap as a background map.
- Align the details of the background map offset.
Step 5 - Align Existing Map
- Use Affine Transform to align the existing map to the background map.
- Mark reference points on both maps and align them.
Step 6 - Rotate Map to Magnetic North
- Use OCAD 2020 to rotate the map from OS Grid North to Magnetic North.
Step 7 - Re-align Layout Content
- Note the Coordinate figures and MN setting from the map.
- Set the layout file to the same values as the map file.
- Draw new North Lines if needed using Map Grid.
Georeferencing maps is essential for accurate navigation and data alignment, and with these steps, it becomes a manageable task.