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 for distances up to 200m.
A mosaic of orthorectified images
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.
GPS coordinates and PPP
Our capture process uses GPS coordinates, which are further refined using PPP (Precise Point Positioning). Because of this, the resulting imagery is georeferenced using ITRF2014 at the epoch of capture.
For example, the epoch of a capture from May 1, 2018 is ITRF2014(2018.329). With ITRF2014 being an earth-fixed datum (as opposed to plate-fixed datums such as GDA94 or NAD83), imagery aligned to ITRF is subject to continental drift. For example, in Australia the movement is 70mm per year and some parts of the US move at about 14mm per year. In order to compensate for the continental drift, we provide plate-fixed projections through WMS and reverse the shift by a variable amount depending on the capture epoch. Those projections are based on either GDA94/GDA2020 (Australia) or NAD83 (US). As a consequence, we encourage you to use one of the plate-fixed projections in order to minimise misalignment in your GIS application, especially using surveys captured years apart.
This information should aid you in correctly georeferencing our imagery. We found that in most cases, it is sufficient to use one of our NAD83 or GDA94/GDA2020 projections to eliminate positional errors due to datum differences.
Cadastral boundaries (AU)
Nearmap uses Precisely maps for displaying cadastral boundaries. The spatial accuracy of Precisely maps in selected metropolitan areas is approximately 2 meters, but this accuracy may extend to 10 meters in certain rural and remote areas. The dataset is as accurate and complete as possible, relying on the precision of the source data, with ongoing updates incorporating additions and corrections to enhance the accuracy of its representations.
NOTE: Nearmap does not bear responsibility for the accuracy or reliability of Precisely data.
Horizontal accuracy of Nearmap Vertical imagery
In line with ASPRS Ed2V2, positional accuracy is quantified and reported using Root Mean Square Error (RMSE). RMSE provides a statistical measure of the deviations between the dataset coordinates and the true ground coordinates derived from an independent source of higher accuracy.
- Horizontal Accuracy (RMSEh): Horizontal accuracy measures how precisely features are positioned within the imagery’s planimetric (X, Y) plane compared to their true locations on the ground.
- Legacy Metrics (Reference Only): Older standards used measures like CE90, LE90, and sometimes 95% levels (CE95, LE95). ASPRS Ed2V2 now uses only RMSE to define accuracy, making it clearer and more consistent. CE90/LE90 values are shown in the table for reference, but RMSEh is the official metric for assessing compliance.
The horizontal accuracy specifications for Nearmap Vertical Imagery are outlined below, grouped by the primary capture system used.
Camera System | Nominal GSD | RMSEh (Horizontal)
| CE90 (90% Confidence (reference only) | ASPRS Class |
|---|
3rd Generation | 4.5 | 14.1 cm / 5.6 in | 21.4 cm / 8.4 in | 15 cm |
2nd Generation | 5.6 | 19.8 cm / 7.8 in | 30.0 cm / 11.8 in | 20 cm |
1st Generation | 7.5 | 25.5 cm / 10.1 in | 38.7 cm / 15.2 in | 30 cm |
Nominal GSD applies in most cases; final product GSD may vary due to operational factors such as, flying altitude.
Where we have historical captures taken with our current generation camera system, subsequent captures taken with our earlier generation camera system are improved to an absolute horizontal accuracy 25.5cm (10.1”) RMSEr.
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.