T. Image Georeferencing

Introduction

This tutorial will guide you through the process of georeferencing a map. To georeference a map or image you need at least 3 tie points. These points should cover as much of the map as possible. In this tutorial we will use a sea chart from the Oslofjord. This chart has some marked positions referenced in WGS84, UTM Zone 33, as tie points.

Exercises

A map about to be georeferenced.

It is important to understand what coordinate system and datum the map is in - or make a qualified guess. Note also that you should be able to determine the coordinates of your tie-points, here encircled in red.
The tie-points can be find in the text file coordinates.txt:

Coordinate system: WGS84, UTM zone 33
Lower left: East: 241073.89 North: 6599354.87
Upper left: East: 240904.64 North: 6601351.49
Upper right: East: 245955.69 North: 6601663.55

Georeferencing step by step

• Make sure the image is selected in the project and right-click and select the Geo Reference command in the pop-up.
  (If it is not visible you will have to open the Toolbox by View > Toolbox).
  

• Then the dialog for setting the coordinate system and a window containing the image is shown:
  

• Set geodetic properties of the chart: WGS84, UTM Zone 33 N.

• Click Next.

• Set Method: to Scale and Translate.

• Set Tie point Coordinate Format: to XY.

• Type in the X = 241073.89 and Y = 6599354.87 for the first tie point, click Image pick and click on the corresponding point in the map.
  

• Change to Tie point 2.

• Type in the X = 240904.64 and Y = 6601351.49 for the second tie point, click Image pick and click on the corresponding point in the map.
  

• Change to Tie point 3.

• Type in the X = 245955.69 and Y = 6601663.55 for the third tie point, click Image pick and click on the corresponding point in the map.
  

• Evaluate the result. The Pixel error represents the accuracy of the georeferencing. The value should not exceed more than 1 pixel on each point.

• Click Finish. 
  
  
• Now the image must be transformed from UTM Zone 33 to UTM Zone 32.