ADS40-II Aerial Triangulation

The majority of North West’s projects require further office data processing (after acquisition) to obtain the accuracy specified for the final deliverables. The process to improve and confirm the accuracy is called aerial triangulation (AT) and involves connecting common visual points (tie points) in the overlap area between adjacent lines or the visual identification of surveyed ground locations (ground control points). The tie points and ground control points improve relative and absolute accuracy respectively.


North West’s proprietary Pixelgrammetry™ process is used for all AT performed within North West. Pixelgrammetry™ was developed and designed by North West specifically for the ADS40 push broom scanner.

After the tie points and ground points are selected in the imagery the final AT adjustment is performed with Pixelgrammetry™ software. This process takes all of the input data; including the post processed GPS/IMU, tie points, ground control and preliminary data strips into a least squares adjustment utilizing several sensors and of unlimited size. Pixelgrammetry™ permits a priori estimates for weighting of the imagery, ground control, tie points and airborne control.

All parameters are treated as weighted observations, ranging from known to unknown, and may be weighted to reflect the a priori knowledge of their precision. This is particularly useful in differentially weighting ground control, compensating for different sources of control and varying precision, as well as using horizontal or vertical control with unknown components.

Pixelgrammetry™ performs an iterative least squares adjustment minimizing the residuals at the tie points and ground control points in accordance with their a priori weighting. The adjusted positions and standard deviations of the tie points and ground control confirm the AT accuracy. Typically the final AT accuracy is < 2 times the imagery resolution.

Digital Orthophoto

An orthophoto is a spatially accurate photographic representation of the ground features. In the orthophoto rectification process image displacements due to elevation changes, misalignment of sensor from vertical, etc. are removed.

Other Pixelgrammetry™ modules complete the other orthophoto processing tasks such as mosaicing, radiometric balancing, feathering, tonal transfer curve application, automatic or user defined seam-lines, geometric feathering and removal of hot spots, vignetting, and colour balancing.

DSM Autocorrelation

DSM autocorrelation is the semi-automatic DSM extraction process using multiple stereo viewing angles from an ADS40 sensor to common points on the ground. The end result is a DSM of the visible surface (top of foliage if present) that must be filtered to provide a DEM.

DSM extraction by autocorrelation is a cost effective alternative to conventional compilation.

Softcopy Photogrammetry

Softcopy photogrammetry is defined as the use of digital imagery in photogrammetry, typically in a computer workstation. Softcopy photogrammetry is very efficient and convenient in that large data sets or maps can be efficiently manipulated, edited, and managed using digital techniques. The ADS40 stereo data can be used in several different viewing software packages; i.e. SocetSet, DATEM, PurVIEW and KLT.

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