How to create an orthophoto

An orthophoto is a rectified (North-oriented raster map with square pixels) scanned photogrammetric aerial photograph with corrections for tilt and relief displacement. An orthophoto is obtained by resampling a photograph which has a georef orthophoto to a georef corners.

Requirements:

a scanned photogrammetric aerial photograph on which you can distinguish 2, 3, or 4 fiducial marks;
known principal distance of the photogrammetric camera;
Digital Terrain Model (DTM) of the area.

1. Creating a georef orthophoto:

You need to create a georeference orthophoto for the scanned photograph.

The process is described in topic How to monoplot, steps 1 - 4.

Tip:

For a correct behaviour of a georef orthophoto, it is essential that you have marked the 'Interpolation' check box in the Properties dialog box of your DTM raster map. For more information, refer to the Raster Map Properties dialog box.

When finished creating and editing a georef orthophoto, leave the Tiepoint editor and close the map window as well.

2. Resampling the scanned photo:

Once you have a georeference orthophoto for your scanned aerial photograph, the only step you have to take is to resample the photograph to a North-oriented georef corners.

Usually, you will already have a georeference corners, e.g. the one on which you rasterized your Digital Terrain Model (DTM). In case you do not have a georeference corners yet, create one, for instance by clicking the create button in the Resample dialog box.

To start the Resample operation:

choose Image Processing, Resample from the Operations menu in the Main window, or
in the Operation-tree, open the Image Processing item, and double-click the Resample operation,
in the Operation-list, double-click the Resample operation, or
in the Catalog, click the scanned photograph with the right mouse button, then choose Image Processing, Resample from the context-sensitive menu.

The Resample dialog box will appear.

In the Resample dialog box:

select for the input raster map your scanned aerial photograph (which uses a georef orthophoto),
select for the target georeference a georeference corners,
choose for nearest neighbour, bilinear interpolation or bicubic interpolation,
type a name for the output raster map,
optionally, type a description for the output map.

For each pixel in the output map, a value will be retrieved from the nearest pixel in the input map, or a value will be calculated from 4, or 16 near pixel values in the input map.

Finally, display the output map in a map window.

Tip:

When you add grid lines to the resampled photo in the map window, the grid lines will be straight and perpendicular to each other.