8. Images and Georeferencing

Georeferencing an map/image

To georeference a map or image you need at least 3 tie points. The general rule is that these points should cover as much of the map as possible.

Tie points are selected by clicking on the point in the map/image and then providing the corresponding coordinate for that point.

Geocap has two methods for georeferencing an image:

1. Scale and translate - usually just need 3 tie points. It is used when you have a high quality map/image and the coordinate system is known. This option will not change or re-sample the pixels in the image. It will simply scale and translate horizontally to best fit the reference points.
2. Morph Image - can be used to force an image/map to fit, using an arbitrary number of tie points. The new image will have a 100% match at the tie points. In the area between tie points, the image will be interpolated smoothly. Using this option, the new image is a re-sampled version of the original image.

Use the Geo Reference command to start the georeferencing process. When executing this command the image will be displayed in a standalone window and the georeferencing menu will appear to the left.

Step 1:

1. Specify the datum and coordinate system of the map/image

   If you don`t know the coordinate system, take your best guess.

   Note that the resulting georeferenced image will be in this datum and coordinate system.

2. Click Next

Step 2:

1. Select the Method (explained above) you want to use.
2. Select the Coordinate format of the points you will provide. Either X Y or a version of latitude & longitude
3. Select tie points:
   1. Make sure Tie Point 1 is selected
   2. Type in the values of the tie point or select a reference point from the graphics window by clicking World Pick and clicking on the point in the graphics window.
   3. Select the Pixel coordinate by clicking Image pick and then clicking on the point in the standalone window.
   4. The coordinates will now be updated in the table.
   5. Provide new tie points by changing active tie point to Tie point x and repeat the steps above.
   6. Make sure that at least 3 tie points has been provided
4. Evaluate the result:
   1. Take a look at the Pixel error column. A general rule is that the error should not exceed 1 pixel, but this is of course depending on the size and resolution of the map.
   2. Click Preview to see the result in the graphics window. Display other existing data from your project (i.e. coastline) to see if it fits.
      

Step 3:

1. When you are pleased with the result click Finish
2. Click OK when being asked if you want to georeference the image.

Vertically calibrate an image

Vertically calibrated images was originally introduced as a way to import paper seismic into the Geocap environment. See tutorial: Vertical Image Calibration.

However, the functionality can be used on any image where all three dimensions are relevant, such as cross sections.
The way this works in Geocap is that the image is attached to a navigation line. If you don`t have a navigation line you can digitize one. In its simplest form, all you need is a line between two points. Read more about digitizing.

Make sure you have the image and the corresponding navigation/digitized line available in your project. Start the calibration process by going to View > Vertical Image Calibration.

The vertical calibration process consists of 3 steps:

1. Input/Output - Provide image and navigation/digitized line as input, and select an output folder.
2. Vertical Calibration - Select 3 vertical calibration points and a cut off range. Note that the image will be clipped above and below the cut off range, so adjust these parameters with caution. The vertical calibration points specify the orientation and scale in vertical direction of the image. The points should have different depth value, and preferably cover a large area of the image. Type the depth value, click the Pick button, then pick a point with this depth value in the graphical window. Use the Focus button to zoom in at the point and pick again for a more accurate assignment of the point.
3. Navigation Calibration - Select at least two knot points in the navigation/digitized line, and relate them to their position in the image. The table contains the points in the navigation/digitized line. You can see the x, y and z values, plus additional field data – if present. Click a number in the leftmost column, click the Pick button and then click on the corresponding point in the graphical window.

Click Save in the menu, and your VCI will be saved to the output folder.

Unlike georeferencing and vertical image calibration, well log image calibration is not an operation that will calibrate the image permanently. However, you can save the settings as an item command, which then will calibrate the image on the fly when you execute the command. Read more about item commands in commands.

Calibrating a well log image

Make sure you have imported or created the well that corresponds to the well log image before you start the calibration process. Read more about wells and Calibration and display of well log images.

1. Right-click the well log image and select Additional Display > Well log image calibration.
2. Browse in the well.
3. Click the Initialize log image calibration button and the image will be displayed.
4. Click at the topmost known value in the log image in the graphical window, click Update cursor and type in top log value. Do the same for bottom value.
5. You can also edit the width of the log, the rotation around the well path and displacement from the well.

Click Execute and the log will be displayed at the well position.