I’m currently working on a project georeferencing (or georectifying) a lot of historic maps published in Porter and Moss’ Topographical Bibliography. I’m georeferencing these maps with ArcGIS basemap World Imagery and as a result have spent many days looking at satellite images of Egypt.

Satellite imagery is a hugely valuable resource, but it can be misleadingly precise. One feature of satellite imagery that isn’t immediately obvious is the problem of parallax. Parallax is the displacement of an object when seen from different positions. It’s incredibly useful in astronomy, but more of a problem in geodesy. Maps provide a vertical view of surface of the earth, flattened onto a flat plane below the imaginary godlike viewer. Satellites (and aeroplanes) fly across the curving surface of the globe taking images as they move. This means that some or all of the each satellite image is taken from an oblique angle and that can produce parallax.

The parallax is really clear in a georeferenced map of the Giza pyramids. In the satellite image below the points of the three Giza pyramids are to the north-west of the points in the overlaying georeferenced map. This is because the satellite was at a slightly oblique angle to the ground of the Giza plateau when the image was taken. As a result, when I georeferenced this map I had to be careful to line up the map with the corners of the pyramids to ensure the best accuracy. If I had used the tops of the pyramids my map would have been misaligned.

Acknowledgements and References

Porter, B, and Moss, R. 1932, Topographical Bibliography of Ancient Egyptian Hieroglyphics, Texts, Reliefs and Paintings III: Memphis 1. Abu Rawash to Abusir. Oxford.

Maps and images throughout this blog post were created using ArcGIS® software by Esri. ArcGIS® and ArcMap™ are the intellectual property of Esri and are used herein under license. Copyright © Esri. All rights reserved. For more information about Esri® software, please visit http://www.esri.com.

All the satellite imagery used is ArcGIS World Imagery. Sources: Esri, DigitalGlobe, GeoEye, i-cubed, USDA FSA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community.