Map accuracy: are we there yet?

The project is focused on the relocation of historic lime mines sites across South Africa. Information we have gathered includes high resolution scans of two maps showing the location of every mine in South Africa open at the time of printing. It is from these maps, along with other documentary sources, that we have selected the areas to visit. As you can see below, the location of the mines is clearly marked with a point and a two letter code relating to the mineral being mined with Ls indicating lime (on a side note I have been keeping a beady eye out for those marked as Diamond and Gold, you never know what they might have left behind!).

© Department of Mines, Republic of South Africa 1959

Given that the maps are of a relatively large size it was not feasible to bring them into the field, which even if we did would prove difficult to glean any particularly useful location data from anyway. The solution to this is to make a digital version of the map from the high-resolution scans. Unfortunately the handheld computers we are using do not have a particularly large processing capacity so loading an A0 map scanned at 300dpi and then trying to use it, isn’t efficient or effective. So, what we have to do is to store the data as a much smaller file so it can be used on the handheld computers. This transformation is quite common when using maps like these in an archaeological context, digitising vector data (lines and dots) from raster data (pictures and photos etc.). Vector data files are much smaller than raster data files and they can hold text information about them which is very helpful in the field and saves a lot of time flicking through notebooks.

By using a GIS (Geographical Information System) package it is possible to create a file displaying the locations of the mines as dots, which we can then load onto our handheld computers and use its GPS function to guide us to them. Oh so simple I hear you say, alas no such luck I’m afraid.

A selection of digitised mine locations as displayed in vector format overlaid on a 1:50000 topographical map in raster format.

The main issue with the maps is their accuracy, and furthermore the accuracy of the points we take from the maps. Firstly it is difficult to say how accurately the cartographer plotted the locations. Secondly the scale of the map is so large that the effective accuracy of any point will only be within 1.5 kilometres, in any direction, of the actual location of the mine.  So, even when we are standing right on top of the point stored on the handheld, the mine could be anywhere in a 9 square kilometre area. That’s where we depart from our GPS system to locate the mines and use our eyes, and feet, to find them (always being careful to avoid falling into sinkholes and the occasional thorny bush).

This was only the first step in our preparation for fieldwork, the next will be covered in a subsequent blog post and will deal with how we use and prepare geological maps for the project.

-AR Reid


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