High-resolution Google Earth imagery is a great resource and one widely used by archaeologists the world over. But with tens of thousands of individual satellite images of most of the planet there is bound to be the odd error. During a recent project I was looking in detail at satellite images of the Theban Necropolis, only to discover that Deir el-Bahri appeared to be suffering from some form of spatial anomaly. In the satellite image, the north-west end of the temples of Hatshepsut and Montuhotep display a distinctive kink, like melted caramel.

I can’t determine the precise cause of this kink in the satellite image without more information. It may be something in the projection of the image in Google Earth. Accurately projecting images of a 3d, ellipsoidal earth onto the flat plane of a computer screen using a coordinate system that covers the entire planet is mathematically complex and inevitably leads to compromises and errors. But given that this kink is within a single satellite image, it is more likely to be the product of a georeferencing or georectification error. A satellite image that is projected in ArcGIS, Google Earth or any other GIS, contains information about the global coordinates of the image that allow it to be precisely located in the correct position within whatever coordinate system the GIS is using. These global coordinates allow the image to be ‘warped’ such that it more accurately presents the surface of the earth as it appears. This is sometimes called ‘rubber-sheeting’, which conveys the process very picturesquely. The global coordinates for georectification are calculated from the satellite ephemeris (the information about where the satellite was and how it was positioned when the imagery was recorded) together with other relevant information, including a digital elevation model. If there is an error in the data, the image can be twisted and warped in an improbable and inaccurate way. And so we have kinky temples – Hathor would surely approve!

Joking aside, this is an important reminder that although incredibly useful and typically highly accurate, satellite imagery can and does contain errors. In this case the inaccuracy is large and obvious, affecting an incredibly famous and well-surveyed site. The most novice researcher would be unlikely to miss such an error or believe that the temples truly bend in this improbable fashion. But even in this case, the warp is much more difficult to discern in the cliffs behind than in the rectilinear temples. Smaller-scale errors, in less well known areas, with fewer structures can be much harder to spot. In satellite imagery, as in everything else, it pays to be vigilant. Never assume that the ‘data’ is infallible.

Acknowledgements

The imagery presented in this blog post was 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