Abstract
Abstract. Topographic maps are produced in Portugal by the Army Geospatial Data Centre, based on a geographical database collected from aerial photography in resolutions from 0.30 to 0.50 m. Each map sheet is revised with an update interval of 10 years or more. Many changes, such as new roads or power lines, would be possible to update with much higher frequency using UAV imagery. Although the nominal scale of the published paper map sheets is 1 : 25,000, the actual positional accuracy requirement is of 1 meter, which is compatible with larger scales. Exterior orientation parameters obtained by UAV navigation equipment does not have enough accuracy for that. This paper deals with a method to automatically improve the UAV exterior orientation, intended to be compatible with the positional accuracy standards of the geographic database. The method is based on matching a point cloud derived from UAV imagery, without GCPs, with a reference DSM obtained from conventional aerial photographs, which were oriented by standard aerial triangulation. Results allowed to improve the UAV data to a positional accuracy of 1 meter, making it compatible with the requirements established for the geographic database.
Highlights
Traditional topographic maps of scale 1:25,000 are produced in Portugal by the Army Geospatial Data Centre (CIGeoE, 2019)
There is a significant difference between unmanned aerial vehicles (UAV) image resolution and the aerial photography normally used for this scale, some efficiency in UAV image acquisition can be obtained, especially if field teams do not need to collect ground control points (GCPs) for image orientation
UAV image orientation is possible with the method described, based on the matching of an UAV point cloud and a reference digital surface model (DSM) surface derived from the initial aerial photos
Summary
Traditional topographic maps of scale 1:25,000 are produced in Portugal by the Army Geospatial Data Centre (CIGeoE, 2019). The revision process is time consuming and in many cases, especially in less populated areas, it may take well more than 10 years Even in these areas changes may be significant because of forest fires and reforestation, which originate many new roads and paths. Aerial photographs for the full map production at this scale are collected with a ground sampling distance (GSD) between 30 cm and 50 cm, flying at heights of a few km and covering large areas. UAVs equipped with RTK GNSS are becoming more common (Gerke, and Przybilla), 2016, and would solve the problem Their cost is significantly higher and their operation is not so simple, normally requiring the installation of a base station.
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