Case Study

Hidatsa Earth Lodges

Nightwalkers Butte, Fort Berthold Reservation, North Dakota

Nightwalker's Butte (also known as Duskwalker's butte) is a fortified village occupied in the late 18th century by a dissident group of Hidatsa. An electrical resistance survey was performed as part of an interdisciplinary archaeological study of this site in partnership with the descendant community.

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A highpass filter is implemented by calculating the local mean in a moving window and subtracting the mean from the original map data. The size of the moving window is adjustable. In this case, a 5 meter radius is used. The result is a new map in which the average "background" resistance of the site has been subtracted. The mean value of the new map is zero.

Highpass filtering of resistance data offers a number of benefits to the mapping and interpretation of resistance data. In addition to enhancing the visibility of small low contrast features, it also creates a resistance map with zero mean. This zero mean map can be thought of as a resistance map containing features which are greater than the local average (the positive values) and features which are less than the local average resistance (the negative values). The zero data regions correspond to areas of no resistance deviation from "background" or local mean.

With this insight, it is convenient to interpret positive data as features with "greater than average resistance," or "less than average resistance." In order to display anomalies associated with earth lodges, the color image was plotted from data from which negative values and extreme high outliers had been clipped.