Orthorectification in a Nutshell

Raw satellite imagery has distortions caused by sensor orientation, variations in topography, and the earth’s curvature. After collecting satellite imagery, it has to be processed to filter out inaccuracies through a process known as orthorectification of aerial photographs. It involves the removal of image perspective (tilt) and relief (terrain) effects for creating a planimetrically correct image.

The orthorectified image contains a constant scale representing features in their accurate positions. It enables the measurement of distances, angles, and areas accurately.

The inputs required for orthorectification are:

1.     Image with accurate sensor geometry

2.     An elevation model of the ground surface in DEM, DTED, or SRIM format.

Imagery Distortions

The distortions come in the form of feature displacement, creating inconsistencies in scaling via image. The distortion levels change throughout the imagery scene. It means that a simple ground control point adjustment cannot nullify these errors. Therefore, a correction process for topographical variations and sensor orientation is necessary to rearrange the displaced pixels to their correct position. The final orthorectified image can be used for direct measurement, extracting features, and other applications that need measurement-ready data.

Accurate Elevation Models Are Crucial

Feature distortion on raw imagery is greatly affected by terrain variation. An accurate elevation model is needed        to calculate the terrain variation effects on the image pixels. This calculation is therefore used to position the pixels to the correct locations. There are free terrain data sources present, such as SRTM and ASTER.  

It is often that the importance of using accurate elevation models for orthorectification goes unnoticed. The reason is the high costs of procuring accurate elevation data and investing in hardware, software, data and skilled professionals. But these things should not be given higher value than quality elevation models. If you focus on orthorectification and geo rectification of satellite imagery in your practice, it needs to be detailed and reliable.

Useful Tips to Find the Right Land Surveying Company

When it comes to land surveying, it is important to find both an accurate and cost-effective solution. When seeking land surveying services, it is imperative to compare multiple quotes for land surveying to determine which surveyor is best-suited to your needs.

However, what should you be looking for in a surveyor for orthorectification of aerial photographs? The following are some of the tips for finding the right aerial survey company India for your projects.

• Getting the right scanning technology

Land surveying projects can be completed either through high-resolution laser scanning or high-resolution photo imaging.

These two technologies are referred to respectively as LiDAR and photogrammetry. Some land surveying companies will provide both technologies, while others might provide only a single service. However, others might be able to blend the two technologies, for even clearer and more accurate imaging.

In general, LiDAR is a more expensive but more precise solution, which is why it is used for producing surveys in areas that may be over-grown or otherwise obscured.

However, photogrammetry is a less refined solution, which is known to provide highly detailed 2D and 3D point clouds but only of what is visible from the air and line of sight.

• Using unmanned aerial vehicles

Drones are well-known for improving the safety of work sites. Unmanned aerial vehicles such as drones can perform Aerial LiDAR surveys without risk to the operators.

Ground survey teams can take a long time whereas aircrews must travel by helicopter and plane. To maintain the safety of aircrew, the air vehicle must remain at a safe height. Comparatively, drones can fly much closer to the ground, thereby capturing more details of the terrain.

This is the major reason why most of the land survey companies are now moving towards drone to improve safety, cost, and quality. Drones are less likely to cause delays in a project and cost less overall because they don’t require a crew to operate.

In conclusion, these above-stated are some of the tips that will help you find the right land surveying company for your project.