keywords: orthophotomap, NMT, true orto
One of the most popular photogrammetric products is an orthophotomap. It represents a layer that is cartometric like any map, while having the qualities of a photograph. It has applications in many fields starting with surveying, urban planning and ending with tourism. [1]
Orthophotomap – is a type of map that is created by processing photogrammetric images into an orthogonal (rectangular) projection, taking into account the parameters of the internal and external orientation of the camera and the slope of the terrain. [2]
Features of an orthophotomap:
Advantages
- Accuracy – Points on the map reflect coordinates in the field with high accuracy.
- Reality – Represents a real and up-to-date representation of the terrain.
- Distortion-free – It is devoid of errors related to camera perspective, terrain, or ground curvature.
- Uniform scale – All objects are presented at the same scale.
- Detail – Thanks to the high resolution of the images, all terrain details can be seen.
- Versatility – They have many different applications in many fields.
Defects
- Elements obscuring others – elements such as trees will obscure elements below them. A similar situation occurs in, for example, agricultural fields, where grain obscures all the land, and in such a place the numerical terrain model created on the basis of the point cloud extracted from photos will be erroneously created, because the surface after the point cloud from photos will get the heights of visible objects: crowns of trees, mountains of vegetation.
- Timeliness – it depends more on when some changes are made in the area, for example, a building is demolished then the orthophotomap is no longer up-to-date.
The process of creating an orthophoto:
– Planning and execution of a raid with the appropriate parameters necessary to obtain a quality orthophoto.
– Preparation in the field of control points located within the boundaries of our raid.
– Creation of NMT, NMPT or a hybrid of both depending on the land cover, level of urbanization and even the height of buildings in the area. It is customized individually as needed. It is created from a point cloud obtained from lidar or created from raid imagery.
– Orthorectification, which is the transformation of photos taking into account any distortions to obtain images in which all points correspond to actual coordinates in the geographic system.
– Mosaicization, involves combining individual aerial photographs into a single whole. At this stage, any differences in the colors and brightness of the individual images are eliminated, as well as their overlapping.
– Georeferencing – placing the orthophoto in the appropriate state system.
– Final quality control and publication. [4]
The process of creating an orthophotomaps:
– Planning and execution of a raid with the appropriate parameters necessary to obtain a quality orthophoto.
A “true ortho” orthophoto – differs from an ordinary orthophoto in one key element – the reference surface. It is devoid of radial offsets resulting from the center projection for objects projecting above the numerical terrain model (NMT), on the basis of which the basic orthophoto is created. These were eliminated by using the high spatial resolution of the numerical land cover model. This ensures, for example, that the roofs of buildings are in the correct position, rather than shifted, “tilted.”[3]
Application:
- Survey material – in justified situations and for only some elements of the EGiB and BDOT500 databases, creation of elevation maps.
- Survey material – for visualization, checking the timeliness of data, presentation of geospatial data, monitoring environmental changes or progress of works of large investments.
- Other – detection of arbitrary construction, illegal dumps, police reconstruction of accident sites, etc. [2]
Orthophotos are invaluable transmitters of spatial information requiring precise situational and elevation data. Thanks to this, they strongly push vector maps out of the market, or serve as a basis for the creation of a very well reflecting the spatial location of objects of the base map. The method of /acquisition of data, as well as their processing itself, require relatively little time and effort.
[1] „Ortofotomapa w terenach miejskich” – Borowiec N. , Zabrzeska-Gąsiorek B. https://yadda.icm.edu.pl/baztech/element/bwmeta1.element.baztech-article-AGH5-0008-0010
[2] „Po co nam dziś ortofotomapa” – Krzysztof Bakuła https://sigma-not.pl/publikacja-104359-po-co-nam-dzi%C5%9B-ortofotomapa?-przeglad-geodezyjny-2017-3.html
[3] „„Prawdziwa” ortofotomapa” – Krzysztof Bakuła https://www.sigma-not.pl/publikacja-132451-%E2%80%9Eprawdziwa%E2%80%9D-ortofotomapa-przeglad-geodezyjny-2021-7.html
[4] https://geoportal360.pl/blog/ortofotomapa-jak-powstaje-i-do-czego-sluzy
