Orthophoto
An orthophoto ( Greek orthós correct, straight, upright) is a distortion-free and true-to-scale image of the earth's surface , which is derived from aerial or satellite images using photogrammetric methods .
An aerial photograph results in distortions of a photographic central projection as well as distortions due to differences in elevation of the terrain and in satellite images distortions due to the curvature of the earth. Analog images can be orthorectified using optical projection methods ( orthophotoprojector ) . Digital recordings are recalculated using digital terrain models and georeferenced using points with known coordinates (so-called Ground Control Points , GCP) .
Orthophotos are offered as geo-referenced digital orthophotos (DOP: d igitales O rtho p hoto). DOP are suitable, among other things, as a map background for geographic information systems (GIS). Together with other map information, such as place names and a coordinate grid, the orthophoto becomes an orthophoto map.
Orthophotos are created in different scales and resolutions depending on the application. The national survey generates orthophotos from image flights with high resolutions of up to 10 cm per pixel. Recordings from satellites or space probes provide a lower resolution. To do this, they cover large areas of the earth or a celestial body.
Orthophotos are very topical because they can be recreated with far less effort and therefore more often than conventional topographic maps. Your coordinate and distance measurement is more accurate because, unlike the map, they are not generalized cartographically .
Applications
Orthophotos are used wherever maps are used. City maps are supplemented with orthophotos for detailed presentation and orientation. Supranational organizations measure the size of agricultural land, e.g. B. to monitor subsidy abuse. In the orthophotos, municipalities find a quickly available, accurate and up-to-date basis for their planning and a geo-reference for statistical databases. GIS use the DOP in various file sizes, scales and subject contents, also in combination as a georeference. The variety of applications has increased explosively since the availability of orthophotos in digital form.
Equalization
Orthophoto principle: rectify the image in such a way that the geometry everywhere corresponds to the scale given on a map in an orthogonal plan projection .
When planning the flight and taking the actual aerial photo, precautions are taken to ensure that the distortions do not become too great. One speaks here of the projective and the perspective distortion.
Projective
The projective distortion is caused by shooting at an angle. It is therefore necessary that the camera is pointing vertically downwards and that the aircraft does not make any tilting movements. The projective effects can be eliminated by knowing the orientation parameters (internal and external orientation).
Perspective
The perspective distortion is caused by differences in terrain height. As a result, the scale is different in different directions, depending on the terrain height and slope, and depends on the camera orientation. The perspective effects can only be corrected with the help of a terrain model.
If the distorted image is projected, the original terrain points (a, b) are transferred to the map in the wrong position (blue projection rays) as shown in the figure. The greater the difference in terrain height and the further away the point is from the image perpendicular (image center), the greater the positional error. This positional error in the image coordinates makes true-to-scale measurement of distances and areas impossible.
If the map plane were to be shifted in height for each individual point in such a way that it intersects the terrain precisely at this point, there would be no perspective distortion and thus no positional errors (red imaging rays). The photographic rectification devices (orthophoto projectors / analog process) work according to this principle. An elevation model is used to control the rectification process, with which the correct elevation is set for each point in the terrain. In today's methods, digital terrain models are used . The close triangular meshing of the terrain model has the advantage that each mesh can be rectified individually and thus a transformation parameter approach is obtained for each. Another advantage is that, due to the small meshes, the radial offset is minimal, since only a very small local difference in height can play a role. A distinction is made between the direct and the indirect method in this approach.
In the case of very tall structures, the distortion is significantly greater at their highest point than at the base, for which the terrain height applies. Therefore, very high structures are only shown undistorted at their base.
Vertical image
For vertical recordings, only the areas close to the center of the image are used, as they are less distorted than points that are far from the center of the projection. The higher the camera is above the terrain, the lower the distortion. Points that are located directly on the reference surface (map level or sea level) are not distorted because there are no differences in terrain height.
Data formats and integration in GIS
Digital orthophotos are offered in various file formats. A widely used format is GeoTIFF , but also ECW and MrSID . The TIF file can also be edited using a conventional photo editing program. However, a GIS application program is required for measuring and for geo-referenced use. Such application programs are available as free downloads e.g. B. via QGIS or GRASS GIS (both for Windows, Linux, macOS).
Sources and application examples
Orthophotos at the German national survey
The geodata center (GDZ) of the Federal Agency for Cartography and Geodesy (BKG) provides central orthophotos from the territory of the Federal Republic of Germany. The Germany Viewer of the Bavarian Surveying Administration, which also contains orthophotos, also offers a good overview of the Federal Republic. In addition, all agencies of the German land survey offer orthophotos for their area of responsibility (almost never free of charge). A free download of test data and many examples are available from Landesvergabe + Geobasisinformation Niedersachsen (LGN) . Completely freely usable high-resolution orthophotos (only a source is required) can be downloaded from the Berlin Senate Department. Older orthophotos are also available during an archive search (up to 1928). In addition to national surveys, many private companies offer aerial image data and orthophotos.
Geoimage-Austria
Geoimage Austria is an orthophoto cooperation association of currently 8 of the 9 Austrian provincial governments with the Austrian federal government, represented by the Land-, Forst- und Wasserwirtschaftliche Rechenzentrum GmbH (LFRZ) on behalf of the Ministry of Life (BMLFUW). This federal-state cooperation serves to create the official Austrian orthophoto for the Austrian state administration and for the agricultural subsidy system. The orthophotos produced by Geoimage-Austria have a ground resolution of 12.5 to 20 cm and are also sold on the free market (e.g. Google).
Austrian aerial photo atlas
The aerial photo atlas of Austria is operated by the Federal Office for Metrology and Surveying (BEV) in cooperation with the state surveying services. Orthophotos were created in black and white (B / W, 1: 2499 - 1: 25001, 1: 2499 - 1: 20001), today in color (1: 2499 - 1: 20001, 1: 2499 - 1: 15001) , as well as modern laser scanning (ALS - Airborn Laser Scanner data 1: 10000). They are available from the Austrian geodata network Geoland and the GIS departments of the federal states.
SWISSIMAGE
In Switzerland, the Federal Office of Topography swisstopo publishes Switzerland's digital orthophotomosaic as SWISSIMAGE. This can be viewed in the GIS browser at map.geo.admin.ch , depending on availability, up to a ground resolution of 10 cm. The time travel function partially enables a comparison back to 1979. In addition, with SWISSIMAGE HIST 1946, an orthophotomosaic with historical black and white aerial photos from 1946 (“American flight”) of all of Switzerland with a ground resolution of 1 m was published. In the GIS browser, the orthophotos can be overlaid with additional information such as geographical names and relief shading .
Other uses
- A geographic information system based on aerial photographs can be found on Google, see also: Google Earth
- Practical application: Measure distances and determine areas interactively in the DOP
- Integration in city portals
- Link to business directories
- Forest damage analyzes
Professional associations
See also
- Analytical photogrammetry
- Map projection
- Measurement camera
- Orthogonal projection
- Rights to geospatial information
literature
- W. Rüger among others: Photogrammetrie , VEB Verlag für Bauwesen, Berlin 1978, VLN 152.905 / 27/78
- Hans Peter Bähr, Thomas Vögtle: Digital image processing - application in photogrammetry, cartography and remote sensing , Verlag Wichmann, ISBN 3-87907-409-7
- Yves Egels, Michel Kasser: Digital Photogrammetry , Verlag Crc Pr Inc, ISBN 0-7484-0945-9
- German Institute for Standardization eV: DIN 18740-3 Photogrammetric Products Part 3: Requirements for the orthophoto , Beuth Verlag, Berlin 2003
Web links
Individual evidence
- ↑ Federal Agency for Cartography and Geodesy: Service Center , accessed on September 29, 2014.
- ↑ Bavarian Surveying Administration: Viewer , accessed on September 29, 2014.
- ↑ http://www.lgn.niedersachsen.de/master/C8863948_N8857851_L20_D0_I7746208.html
- ↑ https://fbinter.stadt-berlin.de/fb/
- ↑ ALS - Airborn Laser Scanner data - year of flight. Metadata record - details ( Memento of the original dated May 6, 2010 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , GIS-Styria
- ↑ Topic list ( Memento of the original from March 4, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. , Geoland → basic data
- ↑ https://shop.swisstopo.admin.ch/de/products/images/ortho_images/SWISSIMAGE
- ↑ https://shop.swisstopo.admin.ch/de/products/images/ortho_images/SWISSIMAGE_HIST_1946
