Introduction to GIS application
The GIS application has been constructed by Daniel Löwenborg and Yunyun Yang of Disr Productions, Uppsala. The report they filed in Swedish when the GIS project was completed is available in its entirity here and it forms the basis of this introductory text.
The goal of this GIS project was to georeference and digitize some of the primary documentation of the investigations at the Alvastra pile dwelling. A total of 169 plans have been georeferenced, 120 from Otto Frödin’s trench, 14 from the Eastern trench, 6 from the Middle trench, 7 from the Western trench and 22 from the Trial trench. The geographical coordinate system used is SWEREF99 TM (EPSG 3006). The piles marked on the plans, a total of 1283 piles, have been vectorized into different layers. In addition to this 37 hearths were digitized, 35 from Otto Frödin’s trench and one each in the Middle and Eastern trenches of Mats P. Malmer’s excavations. Where available, information about the identifier number was added to the attributes of the layer (Härdar.shp).
Conversion of squares and local coordinate system to SWEREF99 TM
Two different local coordinate systems were used in the two excavations. A number of piles with GPS coordinates, 15 in total, were used to fix the coordinate systems used in the older and later excavations. These piles are visible on plan (Plan_s43b.tif) and their SWEREF99 TM coordinates are available. They have also been positioned into the local coordinate system used during Mats P. Malmer’s investigations. This information was then used to relate both local coordinate systems to SWEREF99 TM. To start with the piles on the plan were georeferenced to modern coordinates. Subsequently a coordinate system based on 2 m² grid was created to match the squares used in the older excavations. Then Plan_s43b.tif was adjusted to fit both the piles and the reconstructed coordinate system. The remaining error (RMSE) was 0,23m.
Otto Frödin’s trench
120 plans from this trench were delivered as PDF-files labelled as F1 – F153. They were converted to jpg-format and georeferenced based on the local coordinate system according to the 2 m square grid given on the plan. All the piles visible on the plans, 1057 in total, were then digitized (Pålar_F.shp).
Mats P. Malmer’s trenches – Eastern, Middle, Western and Trial
These trenches were digitized as polygons in the local XY-coordinate system. They were then adjusted to SWEREF99 TM using the relationship between the XY values of the piles and their measured GPS coordinates as described above. When the trenches had been adjusted to SWEREF99 TM (Schakt_SWEREF99TM.shp) all the plans were georeferenced to the boundaries of these trenches so the plans acquired the correct position in SWEREF99. Then the pile plans (PEE2.jpg, PEM2.jpg, PEW1.jpg, PES2.jpg) for each trench were used to digitize the piles in separate shape files (Pålar_Schakt_E.shp, Pålar_Schakt_M.shp, Pålar_Schakt_S.shp and Pålar_Schakt_W.shp). When the trenches from Mats P. Malmer’s excavations had been georeferenced and digitized, there is a difference of about 1 m in comparison with other plans. As it has not been possible to establish the source of this error, no attempts have been made to adjust the two coordinate systems to each other. However, it is necessary to be aware of this difference. If the source of error can be established in the future, all GIS data can be corrected then.
All the plans georeferenced in this GIS application consist of so-called post excavation plans (with the prefix PE in indexation). They are listed here: Planer_georefererade_i_GIS.xlsx.