Orthorectification is an essential process in creating accurate maps of the planet’s surface. And it’s particularly important when you are building simulation and training environments. Let’s see why.
Key takeaways:
- Orthorectification is a process that removes distortions from aerial and satellite imagery
- Without orthorectification, inaccuracies will appear in GIS maps
- Learn about the process of creating orthorectified images
- Find out how we use them when building simulation and training environments
Whenever a satellite (or other remote sensor) takes an image of the planet’s surface, distortion is inevitable. Multiple factors come into play here – the curve of the Earth’s surface and rotation, topographical variations, the planet’s atmosphere, as well as the angle and position of the satellite itself.
To resolve this unavoidable distortion, mapping providers use a process called orthorectification. The aim is to ensure that every pixel on the image appears as if it was taken directly below the satellite – known as the ‘nadir’.
At LuxCarta, we use this process whenever we create 3D maps. And it’s especially important when we build training and simulation environments – because any inaccuracies could be very problematic.
Quick introduction to orthorectification
Orthorectification is a process that ‘corrects’ a satellite or aerial image, so that it is geometrically correct. With satellite images, the area directly below the camera is known as the ‘nadir’. But as you go further away from the nadir, features of the landscape become distorted. For example, a building at the outer edge of a satellite image will appear to slant. This can also mean that distances can be skewed, and the locations of things can be significantly displaced.
Creating orthorectified images is a complex process based on advanced mathematical calculations. It is usually completed using specialised software that is able to ‘place’ features from the image accurately onto map layers.
To create orthorectified images, mapping companies use:
- Ground control points: These are known locations on the planet’s surface (e.g., a mountain). Orthorectification adjusts the image, so it fits to ground control points.
- Elevation model: An elevation model (typically a digital terrain model) provides information about the topography in a particular area. The image is laid over the terrain model to address the problem of relief displacement. Relief displacement happens because the satellite image is flat, but the planet’s surface often isn’t, so landscape features can appear further apart or closer than they really are.
Benefits of orthorectification
Orthorectified images provide multiple benefits:
- Features from the image are placed in their ‘true’ position
- Scale is uniform across the map
- You can use the map to accurately measure distances
- Orthorectified images provide an easy-to-understand layer in GIS mapping, which many people find easier to interpret than, for example, physical maps
Challenges of orthorectification
It can be challenging to create accurate orthorectified images if:
- The terrain model is not entirely accurate
- Errors or lack of information about sensor elevation angle (if you don’t know where in space the satellite was and what angle the image was taken from, this causes problems)
- Low resolution images can make it very hard to identify features and map them to elevation models
At LuxCarta, we use Sentinel-2 imagery at 10m, which means we can be highly confident of the resolution and reliability of the images we use to create orthoimages.
Common uses of orthorectified images
Orthorectification can be used for a wide variety of purposes, including:
- Simulation and training: Orthorectified images provide an accurate basis for building simulation and training environments – see below.
- Urban and infrastructure design: Work out where buildings, roads, bridges and other infrastructure could be placed.
- All kinds of monitoring activities: Track changes in land use, forest cover, changes in coastline and more.
- Disaster response: Up-to-date images can be used to track landscape changes following landslides, hurricanes, tsunamis or wildfires, and aid in recovery.
Why orthorectification is vital for simulation and training
If you want to build a simulation or training environment that is based on the real world, then accuracy is vital. Failing to orthorectify images would make the simulation confusing, and it would be of very little benefit to users.
For example, imagine a commando unit wanted to prepare for a mission using a simulation based on an actual town in the real world that was based on satellite images. If the image had not been orthorectified, then buildings in the simulation would appear at strange angles, distances could be tens, or even hundreds of metres greater or smaller than in reality. Hills that occur in the real world would not be present in the simulation environment.
Related: How the military use GIS
Orthorectified images help with all aspects of creating training and simulation environments:
- They allow you to place objects where they really are
- They make it possible to identify building polygons and extrude them
- They make it possible to accurately depict land use
- They help designers to choose realistic-looking building facades and roofs (using the image as a reference, designers know what colours and features will be most likely)
Orthorectification at LuxCarta
At LuxCarta, we follow a vigorous and robust orthorectification process for all satellite images we use when creating simulation and training environments, 3D maps and RF planning maps. To learn more about how we use orthorectified images in our products, or to see them in action, contact us for a demo today.
