Mini-Lesson 1: Geometric correction of an AVHRR image

Lesson Aims:

The aim of this lesson is to show the limitations of a linear transform applied to an Advanced Very High Resolution Radiometer (AVHRR) image, of the Mediterranean Sea. It demonstrates the need for a quadratic transform to achieve good rectification of the image and the influence of a poor GCP on the rectification process.

Objectives:

You will learn how to rectify an AVHRR colour composite image using linear and quadratic transforms and compare the outputs to a georeferenced coastline image (freely available on the world-wide web) using a formula document.

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Sample images:

A selection of images from this mini lesson are shown below.

Uncorrected AVHRR colour composite image of the Mediterranean Sea and Black Sea with Ground Control Points (GCPs) superimposed.

Master image of the coastline, downloaded from the US Geological Survey MapGen site, with the same GCPs superimposed.

Part of the rectification table linking geographical coordinates of GCPs in the master image with row and column coordinates of the same points in the uncorrected AVHRR image.

An attempt at rectifying the AVHRR image using a linear transform with the master image coastline superimposed to show how this fails due to the curvature of the Earth.

Successful rectification of the AVHRR image using a quadratic transform with the master image coastline superimposed to show how a perfect fit is obtained. Comparison with the uncorrected image shows just how distorted such a large image can be.