Archaeology 101: Relocating Historical Features from Aerial Photography in QGIS
Identifying the former location of historical features can be an invaluable part of designing archaeological investigations, allowing researchers to tailor survey and excavation plans to spaces in which they are interested in, or assess which features might be impacted by development plans. In many cases, no appropriate source of information exists and such features need to be directly identified though on the ground archaeological survey. In more contemporary and document-assisted archaeologies, historical maps can play an important role in allowing researchers to get a sense of where historical features were located. However, depending on the intended purpose of the map there is a considerable amount of variety in the depth and quality of the information they can provide, and some historical features archaeologists might be interested in are often not considered relevant enough to include (outhouses, refuse disposal areas, etc.).
For archaeological sites with occupation periods overlapping with the intensification of aerial photography efforts in the early twentieth century (or later), these photographic records can provide a further means to determine the location of removed or otherwise obscured features. This blog post provides a short guide to creating a GIS shapefile representing the former location of historical structures from past aerial photography, using a example of this process from recent background research for an ongoing Campus Archaeology project. In some cases, a researcher might be able to access accurately georeferenced versions of this imagery, or have the ability to georeference the images themselves, but the process detailed here allows one to work from historical aerials available online but not accessible for download and use within a GIS application.
To follow alongside the steps laid out in the guide below, one will need 1) a source of aerial photography with the ability to measure distance (either through a provided scale or a digital ‘measure’ tool) and determine cardinal directions, 2) QGIS, a free and open-source GIS software available here, and a contemporary satellite imagery layer added to your QGIS project (for help on this last step, follow this guide.
For a recent Campus Archaeology project, we found that an area of campus slated for construction overlapped with the vicinity of two historical farmsteads, constructed in the late-nineteenth century and early-twentieth century, respectively. While available historical plat maps helped to provide the rough location of one of these farmsteads, they provided no extensive detail on the structures and other features associated with its operations. In order to better understand how we might design our fieldwork to investigate spaces of interest to Campus Archaeology, I composed a map of historical features in the vicinity, using aerial photography accessed though the Michigan State University Spatial Data Management Team’s ‘Historical Aerial Imagery’ tool.
Step 1: Identifying stable points of reference.
Look through available aerial photography and compare these to contemporary satellite imagery, looking for non-mobile features or structures that one can confidently identify across available images. Bodies of water are poorly suited for the task due to shoreline change/fluvial erosion, and roads are typically a unstable choice due to realignments and widenings.
Though arboreal features and trees need to be approached with caution for this use, in this case I was able to identify extant Norway Spruce (Picea abies) windbreaks and a single remaining orchard apple tree (Malus domestica) in the vicinity of the farmsteads, that were consistently identifiable in all available aerials between 1938 and the present. These served as the primary points of reference for composing the map of former historical features.
Step 2: Create temporary ‘scratch’ layer
On QGIS, create two temporary scratch layers (a quick guide available here). When prompted to select a name and geometry type, create the first one as a ‘multi-point’ layer and the second as a ‘multi-polygon’ layer. Name each in a way that will enable you to quickly know which is which. Saving the project in QGIS at this point will prompt you to choose whether to save these temporary layers, select yes and provide QGIS with a folder to save the files in (if you plan to keep them).
Step 3: Measure to feature
Turning back to the historical aerial photography, begin to measure out from your reference point(s) towards a feature you would like to map. Instead of measuring directly between your reference point and the feature being mapped—which would involve keeping track of the line’s angle—use line segments aligned with the cardinal directions. Stop when you have reached a corner of the feature (or edge if feature is circular/ovular). Record the resulting values and associated directions in some way, so you can reproduce them later in QGIS.
Step 4: Reproduce measurements in QGIS
On QGIS, with the multi-point layer selected and editable, select the “measure line” tool. Beginning with your reference point, use the measure tool to reproduce the line segments you used to reach the corner of the feature. Once you’ve reached it, use the “add point feature” tool to record the position of the feature.
Step 5: Measure dimensions of structures
For polygonal features, return to the historical aerial and measure its exterior edges starting from the corner you previously measured to. Again, record the resulting lengths and associated cardinal directions of line segments. If features are not aligned with cardinal directions, you will have to keep track of angles at this point.
For circular features, return to the historical aerial and measure an additional line segment from the edge to its radius and record the result.
Step 6: Reproduce measurements in QGIS and add to multi-polygon layer
Returning to QGIS, select the ‘multi-polygon’ layer and toggle editing. Using the measure line tool, reproduce your measurements from step 6.
For polygonal features, use the measure line tool to reproduce your measurements from step 6. Without erasing the measurements, use the “add polygon feature” tool to draw the boundaries of the feature, moving from corner to corner. Right click to complete the shape.
For circular features, use the “add circle by a center point and another point” tool to create the shape. For the first point inputted, use the center of the feature you found when measuring the radius of the shape in step 6b, and for the second, use the edge position. Right click to complete the shape.
Step 7: Save project to keep your progress, and repeat steps 3-6 as necessary to map each feature.
Once you have a few features done, it is a good time to test the accuracy of your work. One way to do this is to measure between corners of two features on both QGIS and your historical aerials. If the results are non-consistent, you may have made an error somewhere down the line. Alternatively, you can begin at one of the corners of a feature you have mapped and work through the recorded line segments backwards towards the reference point and seeing how closely they align.
Step 8: Additional interpretive and stylization steps
With all the locations of former historical features settled, it is worth turning attention to understanding what each might represent. While you can’t reasonably expect to figure out the exact function each feature would had in the past from aerials alone, there is often enough information to make a few preliminary categorizations.
For the completed feature map depicted above, I started by identifying probable residences. While road proximity and its position within a particularly sheltered area of the farmstead were helpful hints, the primary tip offs were the gabled extensions of the roof on the western and southern aspects of the structure (see right-side of photo below)–a trait not commonly seen on non-residential farm buildings. I applied the term outbuilding in the case of seemingly non-residential structures associated with the house to avoid over-interpreting from the limited information available.
Other features were relatively straightforward to identify. The consistent spacing of the twelve trees on the eastern side side of the feature cluster seemed to clearly indicate the presence of an orchard, and a silo seemed the only reasonable explanation for the cylindrical feature adjacent to the large structure east of the orchard.
With informed characterizations of various features established, you can turn to adding this additional information to the layer’s attribute table. Select the layer name in QGIS and right click, and then select the “open attribute table” option. From there add a new field, select ‘text-string’ when prompted to provide a field type, and name it something along the lines of ‘feature category’. From there, categorize each individual feature in a way that makes sense for you and the task at hand.
From here, you can begin to change the appearance of each individual feature category in order produce a map that allows viewers to quickly distinguish between them. To begin this step, double click on the layer name in QGIS to bring up the ‘layer properties’ window, select the ‘symbology tab,’ and change the field that says ‘single symbol’ to ‘categorized’. Below this, select the ‘feature category’ field for the ‘Value’ entry, and then hit the ‘Classify’ button at the bottom of the window, which will automatically generate color symbols for each category. By double-clicking each category entry in the same window, you can customize the color used for each. When you’re done, click on the ‘OK’ button and examine the resulting changes to the stylization of features, going back to the symbology dialog as necessary to make adjustments. A more complete guide to layer stylization is available here.