Parent Topic: GCPWORKS

Introduction

Raw digital images contain geometric distortions which make them unusable as maps. These geometric distortions can be caused by such things as earth curvature, atmospheric refraction, and panoramic distortion. The goal of geometric correction is to correct for these distortions to produce an image with the geometric integrity of a map. A map can be defined as a flat representation of part of the earth's spheroidal surface, such that any measurements made on the map will agree accurately with the corresponding measurements made on the ground. A map should conform to an internationally accepted type of cartographic projection.

In order for remote sensing data to be useful to resource and environmental managers, it must be geometrically corrected (geocoded) and transformed into the standard cartographic projection and scale for the area under study, so that the data can be overlaid in perfect registration with other cartographic information. This state of perfect registration allows the manager to compare cartographic data.

GCPWorks is a standalone application to import image data from various sources, tie it down to a georeferenced image, and perform registration and mosaicking functions. GCPWorks offers an intuitive, powerful interface to all these steps to improve productivity in an experimental or production environment.

There are five sources of GCPs that are supported by GCPWorks: a Geocoded Image of the same area, a Hardcopy georeferenced Map on a Digitizing Table or tablet, a set of georeferenced Vectors, a set of User Entered Coordinates of known locations, and a Chip Database of 1-GCP images.

The distinction between uncorrected and georeferenced images is an important one. The uncorrected image, also referred as an 'input', 'source' or 'slave' image, is the image you wish to geocode. The georeferenced image, often referred to as the 'target', 'output' or 'master' image, is the image you wish to use as the 'correct' image. The georeferenced image is not necessarily corrected and/or georeferenced. Once an image has been "corrected" using ground control points (GCPs), it is said to be geocoded.

Available Types of Data

The vendors of satellite imagery sell data essentially in one of two forms:

GCPWorks only performs the geocoding corrections.

System Corrected Data
Corrections are made for distortions that are quantitatively known in advance. These distortions include those due to the internal sensor characteristics, the instantaneous position, height, velocity, roll, pitch, and yaw of the sensor platform, and the easterly rotation of the earth while the data is being acquired. Corrections for these types of distortions are made at the satellite receiving station. This ``bulk processed'' or ``system corrected'' data has at least one longitude and latitude position annotated on each scene, and is therefore sometimes referred to as ``georeferenced data''.

Precision Corrected (Geocoded) Data
Corrections are made for the residual errors remaining after the systematic errors have been corrected. The bulk processing corrections are based, in part, on tape header information received from the satellite. Since this information is not very accurate, some errors will remain after the processing is complete.

GCPs are used to correct the residual errors, creating a geocoded image. Ground control points can be defined as ``features of known ground location that can be accurately located on the digital imagery'' (Lillesand and Kiefer, 1987). A good example of a suitable GCP is a highway intersection.

It is common practice to superimpose a full latitude and longitude grid on the georeferenced image.

Parent Topic: GCPWORKS
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