12 September 2011

Which way do I walk?

When doing a magnetometer survey, questions arise of which way should you be walking across the grid, and which corner do you start in? Here are some things to consider when making that choice. They may well conflict with each other, so it is up to you to decide how much weight to give each of them.

  • If there is a slope in your field, you will want to walk along it rather than up and down the hill. This not only saves your legs, but will help to avoid stagger in the results.
  • A funny shaped field can affect your choice, and the correct choice will mean less messing about with dummy readings. For example, if you have a rectangular field with many partial grids along one edge, then you will want to walk parallel to that long edge and start in the corner away from it. That way, you will only need to press 'end grid'. Another example, this time with a field which has a straight edge, and opposite to it a very curving edge. Again you will want to start in a corner away from the curving edge, but instead of walking parallel to it, which would result in problems when you hit the edge, walk towards it.  Magnetometers have a feature where you can finish the line and automatically enter dummy readings to the same point on the next line. Starting opposite the curved edge avoids you entering dummy readings at the start of a line. Counting marks on the string to start a line sucks.
  • If you are surveying a field, its present condition may have an effect on which way you walk. It will be easier on the legs to walk along lines of ploughing or stubble than across them. 
  • The archaeology itself can have an impact on which way you walk. If, for example, you are expecting a Roman road crossing your survey area, you will want to walk perpendicular to it for two reasons. Firstly, you will have a greater resolution of readings across the feature, and are more likely to pick it up. Secondly, use of the destripe filter may hide features that are parallel to the way you walk.
  • Converselywise, if you have a known utility pipe across your survey area, walking parallel to it will help stop the extreme readings from adversely affecting the destripe filter across the rest of the grid.
  • The hardware itself may play a part. For example, with the Bartington GRAD601-2, the data collection procedure expects the left sensor to always be on the edge of the grid with the first line that you survey, so picking a direction to walk will restrict you to two out of four corners to start in.
  • Hardware again. Most machines in use today are fluxgate gradiometers, and produce striping in the results. With the Bartington at least, the balancing procedure results in less striping heading north-south than east-west. Filters can deal with striping though, so you only really need to take this into account if you have a flat square field and no idea about the archaeology underneath. 

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