Georeference Images

Georeferencing images in Petrosys.

Petrosys recently introduced an image georeferencing module into mapping. To georeference means to define a location in physical space - establishing location in terms of map projections and coordinate reference systems. It's a great tool which helps you to get your image data in the right spot.

Below we look at a practical example to help you understand the functionality and the tool, defining the position of a bathymetry image within a map. The tool ultimately helps us to better position the image being displayed within the map.

Step 1: Display reference data

Before georeferencing an image, first display relevant data that will be helpful when positioning the image. Typically, this is some form of geographical data that can easily be identified in the image. In the example, the Australian coastline is being used.

This map (above) shows the correct location for coastline displayed as a ‘blue line’.

Step 2: Add the image

Once the reference data is in on the map, display the image file. This is achieved via: Display/Picture/Raster. Hint: when adding the image layer to the map, interactively position the image as close as practically possible to the desired final location.

The image (above) which has no co-ordinate referencing information has been displayed on the map. The interactive georeferencing tool will allow picking and editing of georeferencing ‘tiepoint’ locations to help us get this image into the correct context.

Step 3: Launch the tool

After the image is displayed on the map, select it and right click. From the menu, choose the Georeference image… option to launch the Georeferencing tool. Petrosys will use the coordinate reference system (CRS) of the current Mapsheet. If a different CRS is desired, select it with the lower right hand button marked Output projected CRS before continuing.

Step 4: Add control points

Double click on a point of the image that needs to be georeferenced. A red box with a tick mark inside it will appear. Next, click the position on the map that will be used as the ending location for that point. An arrow will show the direction the image will be stretched. Repeat this step to add at least 3 Ground Control Points (GCPs).

The user interactively creates Ground Control Points (GCP), which relate a geographical position on the map to a pixel position in the image. Here the GCP is shown between the green coastline in the image and the blue line on the map.

Step 5: Save & display the result

When all of the GCPs have been added, select an output file name for the newly georeferenced image. Also choose the type of georeference image format, which includes support for compressed output in the JPEG 2000 and ECW file formats. After that, simply click the OK button. Petrosys will add the georeferenced image to the map as a new map layer.

Below (left) multiple GCP have been set up to help georeference the image. Below (right) the final result ... that’s better! Now the image is effectively aligning with the coastline.