fbpx

Orthorectification

Orthorectification is a post processing technique that improves the horizontal accuracy of your data. By draping imagery over a co-registered model of the terrain below, we are able to shift the data closer to its true location on the planet. Any medium and high-resolution imagery, including those with and without an RPC file, can be orthorectified.

The accuracy achieved in orthorectification can be further improved upon by the application of ground control points. Once ground control is applied to high-resolution imagery, the data typically achieves an accuracy of 2-meter (m) CE90, 3-m RMSE or 1:2,500 NMAS or better. Please read on for more information…

What Does Orthorectification Look Like?

Our Orthorectification Services

Co-RegistrationGround ControlMosaics

It is not uncommon for geospatial datasets to be misaligned even when both have a stated horizontal accuracy. In these instances, we can co-register the datasets to each other, no matter what format they start in.

  • Image-to-Image – matches an imagery layer(s) to an accurate imagery base layer in raster format
  • Image-to-Vector – matches an imagery layer(s) to an accurate vector base layer, typically in shapefile, geodatabase or KMZ format

Ground control points are locations that are visible in your medium and/or high resolution imagery with a known latitude, longitude and elevation. For projects requiring higher positional accuracies, such as in engineering planning, we suggest applying ground control when the data is orthorectified. Apollo Mapping can:

  • Select the appropriate number and locations of ground control points so that you can collect them yourself.
  • Collect the ground control points you need anywhere on the planet.
  • Apply this ground control to your input datasets to greatly improve the accuracy of the final orthorecitifed product.

For projects requiring multiple images to completely cover your study site, a mosaic is the ideal solution. Data can be fused together from multiple dates, imagery sensors and even resolutions to create a single, color balanced file that is easy-to-use in ArcGIS, AutoCAD or any other geospatial application.


Useful information about orthorectification

Input Datasets
We suggest using input datasets with an off-nadir angle of collection that is less than 20 degrees for orthorectification.
RPC Files
An RPC file is typically delivered with georeferenced imagery. It describes the latitude and longitude position of the image’s upper left corner and its mathematical relationship to the rest of the pixels. Many geospatial programs require a RPC file for orthorectification.
Positional Accuracy
There are multiple ways to measure the positional accuracy of orthorectified imagery. One is the root mean squared error (RMSE) which is the average distance actual points fall from their predicted locations. Circular error 90% (CE90) is the distance within which 90% of the points in the imagery fall from their actual locations. The NMAS define accuracy as related to a map’s scale – while expressed differently, operationally these standards are similar to CE90.
Ground Control Accuracy
Ground control should have an accuracy equal to the pixel size of the input dataset or better. We prefer to use points with an accuracy that is at least half the pixel size.
Imagery Mosaics
Mosaicking imagery from different sensors, seasons and even years can create a single file that is easier to use in geospatial software, however users should expect seams where vastly different datasets have been fused together.

Looking for more information on the United States National Map Accuracy Standards (NMAS)?

Download Whitepaper