The resolution of satellite imagery is crucially important to its usefulness to the archaeologist because it directly impacts the features you can see and the precision with which you can georeference other data. For some purposes (such as overviews or larger maps) we might only need low or medium resolution imagery, but in many cases high resolution imagery is essential to our research objectives. During my recent georeferencing project, it was necessary to use the highest resolution imagery I could obtain to locate the historic maps as precisely as possible.
Unfortunately, although you can purchase high resolution satellite imagery from various providers, it is often expensive and the contract comes with quite strict restrictions regarding what you can do with it and with whom you can share it. This makes free high resolution imagery a very valuable resource for the archaeologist, although it does have limitations. There are various ways to obtain free high resolution satellite imagery, but here I compare the imagery provided by three of the most common providers Google Earth, ESRI ArcGIS World Imagery and Bing Maps. Imagery from all these providers can be accessed online in their applications, where you can also access archived imagery through Google Earth historical imagery or the ESRI Wayback app. But if you want to do much more than look at the imagery it is advisable to access it through a GIS. If you are using ESRI ArcGIS World Imagery you can access it through the basemapping layers of ArcGIS, but all three providers can also be accessed through the Contributed Services of the QuickMapServices plugin of QGIS.
Comparing imagery of Dra Abu el-Naga
Dra Abu el-Naga, at the north-eastern end of the Theban Necropolis, is a an important area where 17th Dynasty royals and Ramesside nobles were buried. It continues to be excavated by various teams, including the Djehuty Project, the German Archaeological Institute (DAI) and the University of Pisa and is the subject of several recent publications, concerning its geomorphology and landscape archaeology.
The following images compare three different open-source high resolution satellite images of Dra Abu el-Naga at the same scale. You can also see a more dynamic comparison in this video on my YouTube channel. first satellite image comes from Bing maps. It is reasonably clear at a scale of 1:2000, revealing the structures of the excavated tombs and the open shafts in front of them, but when you zoom in it becomes blurry and the features are difficult to distinguish (from 1:59 minutes in the video).
The Google Earth satellite image is much brighter and somewhat clearer than the the Bing maps image (from 2.45 in the video). It’s worth noting that the Bing maps image appears to be older than the Google Earth imagery. The Google Earth imagery shows tombs in the bottom left corner, which are covered with debris in the Bing maps image (above).
In my opinion the ESRI ArcGIS World Imagery is the best of the three, although it is not as bright as the Google Earth imagery. The tomb structures and shafts are still visible, but the imagery is sharper and easier to understand (from 4:10 in the video). It also stands up to zooming better than either the Bing maps or the Google Earth imagery. It’s worth noting that you may need to use ArcGIS to export ESRI World Imagery. Although its possible to view it in QGIS, it failed to export as an image.
Limitations of free imagery
Bing, Google and ESRI do not own their own satellites, instead they provide imagery from commercial and government sources. These free high resolution images will not be as good as if you purchased them for yourself, but provided we’re aware of the limitations they can still be incredibly helpful.
The first problem with these images is that they are often provided without metadata, so we don’t know which satellite took them or when. Nor do we know the precise resolution, whether the image is pan-sharpened or which electromagnetic bands are included. Based on how clear they appear I would guess that all the images in this blog post have a resolution of 30-60cm, with the Bing maps image being the lowest resolution and the ESRI image the highest. But it is just a guess. Similarly I believe the Bing image is older than the Google Earth and ESRI images, based on the greater exposure of archaeology in the latter, but I cannot say precisely from the imagery.
Another difficulty with free imagery is that it is updated by the provider intermittently. An image that covered your site last year might have been replaced by now with a less useful version, perhaps with cloud cover, dust or an error in georeferencing. Even if there isn’t an obvious error or kink in the georeferencing, you may find that the new image doesn’t match the rest of your data as well. Global imagery providers use a global coordinate system to display satellite imagery, which may be different from the local coordinate system and projection you are using. As a result you may find a feature does not appear in exactly the same location in the new satellite image as in the old one. The difference is not likely to be large and may not be significant for your project, but if you are using free high resolution imagery for survey or other precision tasks, a shift of a few metres could represent a serious problem.
A lot of the value of high resolution satellite imagery is in the multiple multi-spectral bands provided and the opportunities for raster analysis and pan-sharpening. Free high resolution imagery does not provide access to the individual bands, permit bands to be recombined or allow for raster analysis. While such imagery can form a useful basemap for display or georeferencing, it isn’t suitable for various remote sensing applications or analyses.
Don’t forget to reference
Referencing your satellite imagery is just as important as citing written sources, and perhaps more so, because the ‘fair use’ carve outs are much less clear and tested with this relatively new type of data. ArcGIS provides relatively clear instructions on citing their products. ArcGIS, Bing and Google Earth programmes watermark downloads, but these watermarks may not be included when using 3rd party software, such as QGIS. You may therefore need to check relevant information for how to cite Bing and Google Earth maps in your captions. You may also need to include acknowledgements or further references depending on the terms of service.
Acknowledgements and References
The first two images in this blogpost were created in QGIS using Bing and Google Earth satellite imagery respectively. The third image 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.