'Celtic Fields' – Archaeology's stepchildren
Traces of prehistoric farming in Western, Central, Eastern and Northern Europe:
Finding ‚Celtic Fields‘ in Forests
(Continue to Sondages in ‚Celtic Fields‘)

 Normally, the edge walls of plots in forests are hardly recognisable without aids, especially if they are weakly pronounced, if there is a lot of undergrowth or much forest waste. Exceptions are clear terraces on slopes, which can best be recognised by looking from bottom to top. In more level locations, the marginal ridges are most easily recognisable if they are strongly pronounced and they are located in a forest that is free or poor of undergrowth, e.g. an mature beech forest or an older spruce forest. Despite all this, it is always difficult to map the raines if you do not have suitable laser data. Simple GPS devices or the GPS positioning of smartphones are usually not sufficient for this in the forests, unless you have ideal conditions: bare winter deciduous forest, dry and windless weather. Another precondition for good GPS positioning in the forest is that the device is turned on early, preferably in open country, in order to initiate satellite positioning, which can often take a long time in the forest due to the many obstructing branches.
For demonstrations and excursions, but also for the optimal placement of soundings, a simple system is suitable that I adopted from the archaeobotanist Walter Dörfler, Kiel. For this you need at least 10 poles of the same length, which have similar markings at the top that are easily visible even in dark forest, and markings at the bottom, about 10 cm above the bottom end of each pole. Then lay an alignment across a recognised or suspected field ridge, always sticking the poles into the ground at the same distance up to the lower mark. Of course, no tree trunk should be in the way. The distance between the poles is not important, about 4 m has proven to be useful. If you then look from one end of the alignment over the marked upper ends of the poles, field margins of 10 m width and only 10 cm height can still be easily recognised. If the distances are too large, you can add more poles to the alignment. This method also works with strong undergrowth if the poles only overhang it.

Alignment of poles through two rim walls and a plot (connecting line displayed). Odderade, Dithmarschen

It is much easier if you use a GPS-enabled tablet or smartphone in which a map programme is installed, allowing to import your own geo-referenced graphics, which are displayed together with your actual location. Avenza Maps is a free app, which runs on both Apple and Android devices. Locus Map is available for Android devices only. Both apps also run in offline mode, which can be indispensable in remote forests. The georeferenced raster data to convert a LIDAR dataset to must be loaded onto the device in Geotiff or GeoPDF format for Avenza Maps, while Locus Map only accepts raster data in KMZ format (Google Earth). For all ‚Celtic Fields‘ presented here, both data types are downloadable, and I am happy to provide others on request if the needed laser data are of sufficient quality.
The following is a summary of the advantages and disadvantages of the apps mentioned with regard to their use in searching for ‚Celtic Fields‘ (and other archaeological ground monuments):

Both apps also work offline. Locus Map provides offline maps based on Open Street Maps free of charge or at a low cost during installation, which can be helpful when getting there, because common navigation devices often do not show field and forest paths. Both apps allow both a north-oriented display and one in the direction of movement. The latter is not much use in forests, where reception conditions for GPS signals are poor on average, so I prefer the north-oriented display.

Screenshot of Locus Map (free version with advertising displayed at the top). The green antenna symbol at the bottom right indicates that satellite positioning is currently active. Then, a blue arrow is shown at the location surrounded by a blue circle indicating the uncertainty area. It is important that the Map Items icon is shown (here to the left of the antenna icon) so that you can show and hide the laser graphics. A short tip on the specially recessed button at the bottom left centres the graphic so that the location coincides with the cross in the centre of the screen. This must also always be done before marking a point, which is done by tapping the symbol with the cross at the bottom left.

Screenshot of Avenza Maps (free version). Here the operation is clearer because of the sparse functions. Tapping the arrow at the bottom left centres the graphic on the determined location. A blue circle indicates the uncertainty area. Repeated tapping switches from north-up mode to compass mode, which normally makes little sense in the forest because of the unfavourable reception. Tapping again switches back to north-up mode. Once this has been done, a point can be set by tapping on the symbol to the right of it. Be careful when editing points: they can easily be moved irretrievably! The two symbols at the bottom right are not relevant here.

Continue to Sondages in ‚Celtic Fields‘

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