Distance, perimeter and area measurement
When you have displayed a calibrated (geo-referenced) image, you can make measurements of distances and polygonal areas with reasonable accuracy, depending on the number of pixels per meter shown in the lower status bar.
The image must be calibrated in either UTM WGS84-ETRS89, X-Y / E-N or in one of the EPSG grids chosen in the Options menu.
Begin by clicking the calculator symbol on the main menu:
or use the Window menu selection and Measure if you prefer:
A small window containing fields for Distance, Perimeter and Area will pop up:
Move the mouse cursor to it's top border and drag it with the left mouse button so that it doesn't hide areas or features of the image which you plan to measure. Release the mouse button when it is placed appropriately.
Move the mouse cursor to the first point to be included in measurement and click the left mouse button once. Then move to the next point to be measured and click it. The distance in meters between the two points will be shown.
If you want to measure a polygonal area, move the mouse again without holding the button down and click the left button quickly at each point of the polygon until you have almost completed the circuit. When you reach the last point, click the right mouse button over that point, and the perimeter in meters and the area in square meters will be shown. A red and white blinking polygon drawn over the area will be shown.
You can measure multiple polygons after you have finished the first and recorded the values by clicking again on the calculator button on the upper toolbar or clicking on the Window-Measure menu item.
Terminate the measurement process by clicking on the Cancel Selection button:
If you make a mistake during the selection of the polygon, clicking on this button enables you to start again.
In the figures below, courtesy of Otto Braash, an extreme oblique 1993 false-colour image of a neolithic Cursus of over half a kilometre in length near Karden in the Moselle valley, Germany is shown during and after measurement. The two long sides are not equal in length. The first image shows the length of the longer of the two sides, the second image shows the length of the shorter of the two as well as the perimeter length and total area.
About half this site was visible again in 2011 in a high resolution orthophoto from the Rheinland-Palatinate GeoPortal and the visible part of the upper side was measured as shown below.