Georeferencing in Global Mapper

In the previous text, I described how to georeference a construction drawing using QGIS. Although QGIS is widely popular GIS application, I understand that not everyone wants to use open source software. So this time I want to suggest an alternative solution.

I have decided to present Global Mapper this time. I chose it because of the ease of use. In my opinion, it beats QGIS, at least for georeferencing purposes.

This tutorial presents a workflow to rectify (georeference) a PDF file with construction layout.

Open dataset

• Load non-georeferenced map into Global Mapper (File-> Open datasets).
• In the PDF Options window that pops up, choose Import PDF as raster image (1) option and accept it.
• Global Mapper is not able to determine reference coordinates of the image, so it will ask you to what to do next. Select Yes (2) to manually rectify the image.

Georeferencing

• Image Rectifier window opens.
• First step is to choose rectification method (Options-> Rectification Method).

The method is set to Automatic by default. In this case, the program automatically selects the best rectification method. It takes into account the number of reference points entered. If you are not sure which method will be the most appropriate, it’s a good idea to choose Automatic.

• Next, select GCP projection (3). Search projections by EPSG code (4), type 3879 and find ETRS/GK25FIN. Accept changes.

• Use the first window to zoom in roughly to reference points. The middle window is for measuring GCPs.

Adding reference points

• Click in the middle of the cross, enter the coordinates from the map to Easting and Northing fields. Click Add point to List (5) and select OK. Repeat this procedure for all remaining points.

• Press OK to start calculations.
• The result displays in the main Global Mapper window. It’s always a good idea to check how the result corresponds with a real-life situation.
• Load the reference dataset and display it together with the georeferenced map. First, check if the plan is positioned in correct place. Next, set transparency to map’s background (instruction below). In this way, you are able to view information from both datasets at once.
• If everything is alright, export the map (right click on dataset’s name -> Export as…).

Set transparency to the background

• Right-click on map layer and select Options. Go to Display tab.
• Check the box next to Transparent and click Select Transparent Color (6). Now select the colour which you want to set to transparent (white in this case)
• Save/Accept changes

What to read next

• Check how to georeference an image in QGIS
• Keep up to date with new QGIS 3D visualisation features
• Find out what are the advantages of web-based point cloud viewers from this text.
• Learn 8 tips to choose the best drone mapping software.

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