Screen Imp:
Tutorial 7

How do I join two map sections into one complete map?

A particular area of a map has been split over two sides of a page within our map book. We would prefer to see the whole area as one map, and would also like to produce a printed version.

To do this, we started by scanning each side of the map book page. When spread open, the map book is A4 in size, each page being A5 (half). To produce images in a high resolution, the scanner was set to 600dpi and 24bit colour. The two A4 scans were saved as 104Mb Bitmap files. Performance with such large images is reasonable with a 2GHz Pentium 4 machine.

Next, cropped versions of the images were made using Screen Imp's drag box tool. The cropped images were made slightly larger than the required area.

Even though great care was taken to get the scans straight, the maps appear rotated through a slight angle. So the next step was to rotate the images back slightly so that the map's contents became parallel with the screen edges. The high resolution of the images enables small rotations to be made without causing jagged edges. We use the protractor tool to measure the small angle that the image is offset by.

Starting the protractor tool provides an isosceles triangle with one side parallel to the screen's horizontal edges. The top left corner (with the double arc outside the circular pad) starts off fixed to 90 degrees, so it is double-clicked to remove the constraint. The top edge (line straight through pad) starts off fixed to the screen's horizontal edges, and can be left as it is since we need our horizontal reference.

Screen Imp Protractor Tool Screen Imp's protractor tool, just started

One end of the horizontal reference line is positioned over a horizontal line on the left of the map. The other end is moved to the right hand side of the map. This appears just above the map's horizontal line, since the scan was not quite parallel.

The last corner of the protractor is positioned on the map's horizontal line, to the far right below the second corner pad.

Protractor Tool Corner Pad Protractor top right pad positioned to the right

The middle of the protractor shows the top edge still parallel to the horizontal edges of the screen.

Protractor Tool Line Pad Protractor top edge pad fixed horizontally

This leaves a small angle showing on the left side, giving the angle that the map is offset from horizontal. The angle measured is 34 minutes of arc, or 0.57 degrees (34/60ths). The rotation in this case should be anti-clockwise (negative).

Protractor Corner Pad Protractor first corner showing small angle

Both maps were rotated using the "Add Border", "Rotate Image" feature from the thumbnail view window.

Add Border Form Rotate Feature Add border, rotate image feature

One map also needed rotating 180 degrees to the other, so that the correct sides could be joined together. The drag box tool was also used to measure a datum point into the map edges to calculate the correct alignment when joined together. Finally the two new map halves were joined together with the stitching tools from the thumbnail view window.

The map can now be viewed and printed with Screen Imp's print preview facility. Since the map is of a high resolution, a central portion can be copied with the drag box for a close up view. The horizontal join can just be made out in the centre.

Print Preview Print preview central map portion
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