This guide includes the following topics:
How is Nearmap's Imagery Georeferenced?
The MapBrowser's measurement tools use appropriate local representations of the Earth's surface and are typically accurate to +/- 15cm
Nearmap's Vertical Imagery is made up of mosaics of orthorectified images. This means that each raw photo that goes into making up our imagery has had the full orthorectification process applied, including removal of terrain distortions, lens distortions and so on. Individual orthoimages are then merged together into a mosaic, which is the map image that you see on MapBrowser. We generate high resolution elevation maps as a necessary part of the orthorectification process (so that terrain distortions can be removed).
Our processing system can handle difficult situations such as cloud and smoke, and it has a far lower blunder rate than older orthophoto solutions. Our imagery processing is fully automated, and designed to create Nearmap imagery anywhere in the world, without requiring any ground control point data at all. This also allows us to capture accurate vertical imagery in adverse conditions where ground point data is not available, is obscured, has moved or is inaccurate.
What is the Horizontal Accuracy of Nearmap Imagery, and How is it Determined?
Our current generation camera system captures data with a ground sample distance (GSD) of 5.8cm (2.3”) in vertical imagery with a horizontal accuracy of 25.3cm (10”). Absolute horizontal accuracy is expressed as the root mean square of the radial distance error, also known as RMSE.
An earlier generation of our camera systems captured data with a GSD of 7.5cm (3”) vertical imagery with an absolute horizontal accuracy of 61.7cm (24.3”) RMSE.
Utilizing the next generation data to improve the earlier, the horizontal accuracy of the earlier version is now at an absolute horizontal accuracy 38.7cm (15.2”) RMSE. In other words, since we are continuously capturing imagery and introducing new camera systems with greater levels of accuracy, we make sure we use the gains from the new cameras to make significant improvement in accuracy for historical captures.
Nearmap also uses ground control points to verify our accuracy claims. Characteristics of our ground control points include good visibility, precise location, accurate location and open ground location. In essence, this means that the sample of ground points used are clearly identifiable in our aerial imagery without ambiguity, have a high accuracy of less than 2cm and are not subject to terrain distortions.
Do the Maps use True North or Magnetic North?
The MapBrowser uses true north rather than magnetic north. This is a natural consequence of the EPSG:3857 Mercator projection used in the MapBrowser and similar online maps. Microsoft has a good description of this projection here.