How to georeference images in Micromine Origin

Georeferencing an image links the pixels of the image file to a real-world coordinate system, and once an image has been georeferenced it can be displayed with other data on that coordinate system. Micromine supports georeferencing by most GIS, image processing and CAD software.

It is also normal to acquire an image with no georeferencing, such as a scanned historic map. This kind of image can still be georeferenced if three or more locations can be identified on the image and their real-world co-ordinates are known. Such locations are known as georeference control points (GCPs).

To georeference an image, select File | (Image) Image | Image Georeference from the ribbon or click the Georeference button on the Vizex Image dialog. In the first option, you will need to select an image file.

The Georeference Image window appear, comprising two panes:

• Image: Displays the image to be georeferenced.
• Vizex: Optionally displays the current Vizex view as an external coordinate reference, which should cover the same region as the image.

The ribbon includes options for adding, moving and deleting GCPs, along with tools for manipulating and configuring the Vizex view. Snap Mode is available in the Vizex pane for precise GCP picks. Beneath the panes is the Control Points list, which lists the image and real-world coordinate values along with their errors.

To add a GCP, click a feature in the Image pane whose location you know. It should be an obvious corner, surveyed structure (e.g. drillhole collar), or map element (e.g. intersecting gridlines). Next, supply its real-world coordinate values: for an image with no Vizex view, manually enter the values for each GCP. For an image with a Vizex view, simply click the corresponding feature in the Vizex pane. Once two GCPs are selected, the panes are geolinked and a preview of the georeferenced image is overlain in the Vizex pane. Adjust the slider to change its transparency.

GCPs always require 3D coordinates; to georeference an image on an orthogonal plane (plan, cross-section or long section), enter a Section or Elevation value in the Vizex pane and lock the plane. This forces the locked axis to always take the Section or Elevation value.

Only three GCPs are needed for a georeferencing solution. However, adding more will improve its accuracy. Once there are four GCPs, Micromine will measure the error and predict the real-world coordinates of each GCP. You may optionally disable any GCPs with a large error.

When you georeference an image, Micromine creates headers for Mapinfo and ArcGIS along with a Micromine header. Headers have the same name as the image, with a GRF (Micromine), TAB (Mapinfo) or xxw (world file) extension. For a world file, xxw represents the first and last letter of the original image extension followed by a w, for example tfw for a tif image.

Micromine does not rectify image defects like folds in a scanned map or geometric distortion in an airphoto. If your image is distorted, you must rectify it, which also georeferences it, in a GIS or image processing application.

This video shows the process of georeferencing an image in Micromine Origin.

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