What is an orthophotomap? – definition and features
By definition, orthophotomap is “a photographic map produced by differential processing of aerial images”. This definition can be hard to digest, so we’ll take a closer look at it.
What is the difference between an orthophotomap from an aerial image?
Generally, orthophotomaps are created as a result of processing of satellite, aerial or drone photos. Main difference is the change of the projection (perspective) from the central projection to the orthographic projection. Orthogonal, or perpendicular to the photographed plane.
Hence the name: “ortho” – from the orthogonal projection, “photo” – from photography, “map” – because the final product may be used in the same way as a map and it has some of the features of a map – scale, coordinate system, proportions and can be used to perform measurements.
Central projection (where the center is a single point – camera sensor) causes the apparent space curvature and non-uniform scale. The larger the photographed area, the differences in terrain, the shorter the focal length (wider view angle of the lens), the greater the distortion.
An extreme case can be seen with a fisheye lens, where the distortion is intentionally not corrected.
In the orthogonal projection, every point of the plane is photographed perpendicularly, vertically from above. To achieve this, ortophotomaps are created by taking hundreds of photos of the area of study, and then combined in the process of orthorectification and mosaicking.
Objects that stick out above or below the surface of the orthophotomap may become bent or slightly shifted – in their case, the scale will not be maintained. They might also obscure objects behind them.
The use of drones and photos taken at a really high overlap allows to combine photos so that the contours of buildings (or other high objects) are projected exactly on their basis. In this case, we speak of a “true ortophotomap”
Accuracy of the ortophotomap – GSD
Accuracy and the detail of the ortophotomap mainly depends on its resolution – Ground Sampling Distance (GSD)
- A GSD of 30 cm/px allows for the identification of ground elements – buildings, roads, etc.
- A GSD of 10 cm/px allows for a more precise inspection of urban areas – distinguishing between road lanes, paving slabs, building boundaries and plots
- A GSD of 5 cm/px is very detailed and allows for inspection of roofs, gutters and tiles, distance and volume measurements. Additionally it allows you to distinguish between individual people.
- A GSD of 1 cm/px characterizes the most accurate orthophotomaps, on which the works submitted to ODGIK can be based on.
These resolutions are much more accurate than those found in traditional photogrammetry. Information and measurements provided this way allow (partially or completely) to resign from further geodetic measurements
Orthophotomaps – applications
Orthophotomaps are used wherever precise and accurate mapping of the area is needed (for measurements or visualization). Although there is a wide spectrum of applications, two main trends seem to dominate – maps for civil engineering and maps for environmental analysis.
Management of a construction project and promotion
Digital maps can be helpful at any stage of a construction or instractural project. From the presentation of the tite and visualization of development plans in front of investors, through the inventory of selected plots, sale of land to promotion of the completed project.
As part of such promotion, marketing campaigns, to present a given place of facility, can be implemented. Create a visualization of the area e.g. in tourist and ski resorts, present available routes, trails and viewpoints.
Civil engineering – construction work planning
Orthophotomaps are used in the design and implementation of construction works. The work progress is monitored without having to stop it. Subsequent stages of construction are evaluated on a cyclical basis. After completion of construction, an as-built inventory is carried out, e.g. by comparing performed work with plans and orders.
Map of the terrain is also used during planning works, construction and inventory of infrastructure such as roads, railroads, transmission lines, power lines and gas pipelines
Inventory of pits and landfills
In the case of objects such as pits, landfills, warehouses or open-pit mines, 3D models of these industrial areas are created. They are used for measuring, planning and monitoring. The volume of excavated material, stored material and stockpiles are measured. The size (volume) of delivered bulk material and the compliance of deliveries with orders are compared. The volume of earth masses are measured and the cost and time of disposal is estimated, e.g. from the construction plan
In this way the rate of growth of landfills, scrap yards, etc. is tested.
Land and building registry
For the purpose of land and building records, digital orthophotomaps are used to determine plot boundaries, measure their area and length.
References to other maps
It is also used to update older, existing maps or to compare maps of different types (this way, unauthorized construction sites or illegal waste dumps can be detected).
A raster (photomap) can be overlaid with a vector map e.g. of roads, buildings, powerlines
A CIR (Color-InfraRed) thermal imaging map can also be created. It is a map made of infrared photos. Such maps are used to study, for example, the condition of plants, forests, humidity of soil, detection of underground pipelines.
Supervision of field changes
Cyclically created maps of a given area allow for monitoring and detection of changes in the natural environment. Parameters such as the occurrence of landslides, the level of surface water or type and density of existing vegetation can be monitored.
Contrasted with the changes in the natural environment, plans for the development of the settlement network (urbanization) can be presented.
Digital orthophotomaps in environmental analysis allow for monitoring of field conditions, forests, rivers, lakes, land etc. In forests they are used to study the condition of trees, detect damage, the occurrence of pests, or to monitor biodiversity of the area
Documentation of natural disasters
In the event of natural disasters such as fires, floods, droughts, it is possible to study their progress and document the extent and type of losses.
Drones with a multispectral camera are also used in agriculture. Such orthophotomap of crops allows for optimization of fertilization, verification of crop growth, inventory of damages or study of soil melioration
Sometimes digital terrain maps are used for unusual applications such as archeological exploration, which would be difficult to do from the ground.
Digital orthophotomaps from a drone – planning a photogrammetric flight
To put it simply, using a drone with a camera, hundreds of photos of the mapped area are taken. They are then processed and assembled into a single map.
The first step is to plan the flight and choose the appropriate parameters. Depending of the purpose of the mapping, the following is determined:
- type of drone – fixed wing for linear objects (e.g. roads, rivers) and large areas or a multirotor for small, local areas, like construction sites surveys
- camera and sensors resolution, depending on the required pixel size – GSD,
- the amount of side- and overlap – reaching up to 80% for precise terrain models and true orthophotomaps
Ground control points
Next the photogrammetric network is created. It is a collection of so-called photopoints, i.e. points that can be clearly identified (along with their coordinates) both in photos and in the field. These can be special signs painted on the ground or naturally occuring points such as manholes and central points of roundabouts.
Performing the flight
Flights are planned in a special program that controls the parameters of the aircraft and controls it during flight. At this stage, hundreds of photos are captured together with geoposition data (GPS) and the orientation of the camera in space – so called elements of the external orientation (IMU – inertial measurement unit)
In the next step, a computer algorithm performs a process called matching based on the GPS data, elements of the external and internal (focal length, sensor resolution, photo distortion) orientation. It involves finding common points, called tie-points and collecting them between points.
Next, another iterative algorithm combines a set of images into a geometric network. It uses the combined images, information about the camera autocalibration and ground control points to orient the whole set in a known coordinate system using GPS data.
On this basis, a DTM – Digital Terrain Model – is created.
Digital surface model DSM (or in the case of photos with infrastructure and vegetation, DTM) is not yet an orthophotomap. The next step is to superimpose images on the previously obtained digital surface model (i.e. change from the central projection into an orthogonal, perpendicular one) na mosaicing (i.e. combining multiple map fragments into one).
Advantages of orthophotomaps taken from unmanned aerial vehicles (UAS/UAV)
One of the parameters of the photogrammetric flight is the density of side- and overlap. In practice, it means how many pixels of two consecutive images are shared. The greater the overlap, the more densely the images are taken.
With an overlap of about 80%, a true orthophotomap can be obtained, in which there is no perspective distortion of tall objects and no obscuring of parts of the terrain. Objects are projected perpendicular to their bases.
Better than satellite images
Drones take photos from a height of 50-500 meters above the ground. Additionally, flights are planned with the weather forecast in mind. In this way, cloudiness and fog can be avoided and high air humidity at such a distance does not disturb colors.
The cost of making such a map is relatively low, and the results can be obtained in 24 hours.
The high mobility of drones allows them to perform flights nearly everywhere and anytime. The main limitations are only the weather conditions (wind speed, rainfall), possible take-off and landing sites and no-fly zones.
Working in difficult conditions
Drones allow for the possibility of working in difficult or unreachable terrain for a surveyor. For example, over open-pit mines, construction sites, or rocky or swampy areas. In the case of flights and mapping of construction sites, there is no need to stop work.
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