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Grid representing a field near Stettler, Alberta, Canada.

Location: near Stettler, Alberta, BC, Canada
Type of data: Somewhat cleaned GPS data.
Easting: 363,000 (in meters wrt NAD83)
Northing: 5,791,000 (in meters wrt NAD83)
Elevation: (in meters wrt NAD83)
Number of points: 19,535

This data was collected from GPS units mounted in farm equipment. A small field (less than 1/4 section) may yield between 10,000 and 100,000 points, and it is asymmetrically sampled. Typically a piece of farm equipment working in the field travels at a couple of meters per second, and may be 10 (or more) meters wide. So "strips" of points are collected where the density of points down the track is about 10 times the density of points perpendicular to the track. Sometimes there is more than one combine working the field, so there may be a multiple of the combine header width between passes down the field. Other sources of missed sampling are wooded areas, sloughs, and weed patches.

The density and distribution of these data points can present problems for gridding programs.

Generated grid including the original data points(in red).

Grid of field showing original data points.

The default settings in QuikGrid for a dataset like this will tend to leave the unsampled areas blank. In some cases, for example, where the data represents marine depths and the unsampled areas represent land masses, this may be desirable. Two parameters in QuikGrid may be adjusted to fill in the unsampled areas. They are under "Edit... Grid Generation Options". For this data a "Distance Cutoff" parameter of 300 and an "Edge Sensitivity" parameter of 300 provides the fill shown in the above image.

Increasing the distance cutoff to 1000 fills in the gaps for this data. The following image shows contour lines every meter with the bold contour lines representing lines which are multiples of 5 meters. The original data points have been omitted and colour has been included. The low ground is blue in colour and the high ground is red.

Contour map with blank areas filled in.

Image after processing through ER Mapper.

Acknowledgment:

I would like to thank Tom Goddard, Precision Farming Project, Alberta Agriculture, Food and Rural Development for the data set.

Created: November 2, 1996.
Revised: November 29, 2001

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