In a previous post I described how I georeferenced a difficult map of Abu Rawash. During that process I had to ignore the pyramid of Djedefre because it was drawn at a different scale to the rest of the map. Here and in this video I discuss how I subsequently georeferenced the pyramid of Djedefre at the correct scale.
The map of the pyramid of Djedefre was cropped from map I in Porter and Moss’ (1932) Topographical Bibliography volume IIIi, which is featured in my previous post.
First, I identified the archaeological features in the map and the satellite image. Although the pyramid is very clear in the satellite imagery, this process was complicated by the long ramp or causeway heading north from the pyramid. Initially I assumed this was the pyramid’s causeway to the Valley Temple, but the angle of this feature in the map, and the angle in the satellite imagery were different. This made me wonder if the feature I could see in the satellite imagery was some kind of recent Decauville track for removing spoil from the pyramid, and the actual causeway ran at a different angle and had been removed. Having georeferenced the map I think the feature is probably the pyramid causeway in both cases, the angle in the map being incorrect due to the differences in scale between the drawing of the pyramid and the rest of the map. If you review the image of the georeferenced map in my previous post you will see that the end of the causeway as drawn in the map lines up with the end of the feature in the satellite image, even though the pyramid is incorrectly drawn. This suggests that the angle of the causeway became misaligned when the pyramid and the rest of the map were joined together. Nevertheless I chose to ignore the causeway when georeferencing the pyramid, because its questionable accuracy would make georeferencing more difficult and it was readily visible in the satellite image anyway.
I then scaled the image to the correct scale to align it to the satellite imagery (from 1.20 minutes in the video). As I note at 4.59 in the video, one thing to be aware of when georeferencing maps is that the lines of the map occupy space within the GIS – so the line of the enclosure wall of the pyramid complex represents 3m on the ground after georeferencing. This can make it difficult to align the map with satellite imagery, particularly if the map only covers a small area and/or is cropped from a much larger map.
Once I determined approximately the correct scale (1:2250) I began linking the ground control points in the map and satellite imagery (from 4.15 minutes in the video). This revealed further inaccuracies and forced me to make decisions about which points in the map I believed were more accurate than others. In this previous post I discussed the importance and limitations of RMSE. The georeferencing of the map of the pyramid of Djedefre really emphasises how RMSE and residuals can be used to improve georeferencing, and also the limitations of the process. I used the RMSE and residuals, combined with the visual position of the map on the satellite image, to test the ground control points (from 10.08 in the video). They rapidly revealed that parts of the pyramid complex had been drawn inaccurately in relation to each other. After noting that the satellite pyramid and south-west corner of the enclosure were in dashed lines, I opted to set the ground control points elsewhere as it seemed likely that the satellite pyramid was more speculatively drawn. Testing various ground control points also revealed that the enclosure wall around the complex was drawn closer to the pyramid than it really is, forcing me to chose whether to include the complex enclosure wall in the ground control points or concentrate on the pyramid. I chose to focus upon the pyramid and mortuary temple, and rectified the map with an RMSE of 3.59, which was an improvement on a previous attempt, but still far from the 0.75m RMSE which would represent the 1:3000 ideal (Conolly and Lake 2006, 82-83). The inaccuracy in the map, its scale and the resolution of the satellite imagery are all contributors to this high RMSE. Depending on what I need to do with the map, I may seek out a more recent map or re-georeference it. Georeferencing a map this small, with this many inaccuracies, to satellite imagery, is always going to be difficult and likely to produce a high RMSE.
Acknowledgements and References
Conolly, J. and Lake, M. 2006. Geographical Information Systems in Archaeology. Cambridge.
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.