Exercise : Measuring heights of trees outside the forest (TOF) in digital orthophotos (DOP)

Shadows in orthorectified remote sensing images

Figure A: Shadow length s and object height h

A simple application of the intercept theorem in elementary geometry is to determine the height of a tree by measuring the shadow length. According to Fig. A the shadow length \(s_1\) and the height \(h_1\) of a man-made object such as an overhead line tower were measured on the ground. If the lenght of a tree's shadow \(h_2\) is determined at the same time of the day we are able to compute \(h_2=s_2*\frac{h_1}{s_1}\)

We may also use trigonometry if we know the sun elevation \(\alpha\). The height of a tree is then computed \(h_2=s_2*\tan \alpha\)

Measuring shadow length and sun azimut in digital orthophotos

Load and display German Geoadata using QGIS 2.4 following the Exercise: Displaying German Geobasis data (GBD) or load the saved project file .\GBData\display_gbd.qgs where the project coordinate reference system (CRS) is set to ETRS89/UTM32N (EPSG:25832).

1. Create a new shapefile layer
   1. Select Layer --> New --> New Shapefile Layer.
      • As layer type, select Line. Click the Specify CRS button and select ETRS89/UTM32N (EPSG:25832).
      • To add an attibute:
        • For the attribute's name type Class into the Name field of the New attribute section.
        • Select Whole number as data type.
        • Confirm with Add to attributes list.
      • Confirm with OK and enter path and name (e.g. measure_shadow_length.shp) in the following menu.