Control points to improve the accuracy of creating orthophotos

Ground control points make it possible to achieve sub-metre georeferencing accuracy for satellite images and centimetre accuracy for aerial photographs, and to eliminate systematic distortions of onboard equipment.

The geopositioning accuracy of a satellite image from RPC coefficients is generally lower than the accuracy of creating an orthophotomap of the corresponding scale. The planimetric accuracy of a very high-resolution satellite image from RPC coefficients is from 7 to 30 m depending on the imaging equipment. When one ground control point is measured, orthophotomap georeferencing accuracy can reach up to 1 m.

At the preparatory stage a planimetric and elevation control project is drawn up, survey boundaries and zones for identification marks are defined.

On the ground, in the project zones, identification is performed and clear contours that are confidently interpreted on images of the corresponding resolution are selected as marks: power-line pylons, corners of fences, curbs and barriers. In the field, less distinctive objects may be used — solitary trees, large rocks. For a new survey, artificial identification marks with high contrast relative to the terrain may be installed.

Well-visible static objects are selected for control points: building corners, road markings, railway crossings. The more points there are and the more evenly they are distributed, the higher the orthophotomap accuracy; the number depends on the project scale, terrain and required accuracy.

Geographic coordinates of the selected objects are determined by geodetic methods and equipment — GNSS receivers. Dual-frequency receivers with GPS/GLONASS navigation systems are used most often. Measurements are performed with high accuracy. The scope of work includes measurement control.

Issue alignment, analysis of remote sensing data availability, additional and reference data — from 1 to 5 days*. Contract signing — from 1 to 5 days*. Image acquisition — from 2 to 10 days**. Photogrammetric works design — from 2 to 5 days*. Determination of control point coordinates on the ground — from 15 days*. Digital orthophotomap creation — from 2 days*. Preparation of a technical report — from 1 to 12 days. Total in the table — from 10 days.

* working days. ** from the date of receiving 100% advance for remote sensing materials. Timelines depend on the number of km² and product type, availability of archival remote sensing materials, additional and reference materials, area and configuration of the territory, remoteness and transport accessibility.

Using archival and new high and very high-resolution satellite images with high-accuracy orbital georeferencing makes it possible to obtain orthophotomaps with high accuracy and detail without ground topographic and geodetic works to obtain control points. Satellite data are obtained more promptly because they may already be in the operator archive.

Using aerial images and UAV/drone images to create orthophotomaps requires a new survey, which takes more time because of special permits and a site visit. At the same time, orthophotomaps from aerial photographs have high visual informativeness and measuring properties.