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Winter 2005/2006
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GIS and Surveying

Where Did That Geospatial Data Come From?

  click to enlarge
This map shows surveyed points with precise building locations overlaid on a DOQQ.

Surveyors are the backbone of geospatial data accuracy. GIS professionals generally think of geospatial data as being derived from a series of points, lines, and polygons. But if the data that comprises coordinates or basemaps is traced back, it will be found that a surveyor usually had a hand in it.

GIS professionals think of these points, lines, and polygons as being derived from coordinates. Surveyors think quite differently. Surveyors think of their data as comprising a series of observations or measurements. Surveyors take a series of measurements—angles, distances, radio wave phase shifts for GPS, etc.—with a variety of instruments, including steel tapes, electronic distance meters, GPS receivers, and theodolites.

Surveyors go to great lengths to protect these original measurements (e.g., note the shelves of old field notebooks often found in surveyors' offices). These measurements are compiled and adjusted to best distribute the minor errors that naturally occur in any measurement, then the final coordinates are produced. Many new technologies have taken a long time for surveyors to adopt because of the difficulty of storing and reproducing the original measurements.

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Surveyors can use GIS to store, manage, and display control points. This map was created as a quick reference for available vertical control. The control and the orthophoto displayed are both publicly available.

When GIS professionals get coordinates from surveyors, whether to control orthophotography or property line locations, those coordinates are generally developed from adjusting a series of measurements. One significant product of this measurement adjustment is knowledge of how accurate the resultant coordinates actually are. This metadata on the positional accuracy is often not stored or even delivered with the coordinates. Had this information been transmitted and saved with the original data, the original measurements could have been reused in the future.

What happened to these original observations or measurements? What happens if more measurements—possibly more accurate measurements—are taken in the same area? How do GIS professionals use this new information? Answering these questions leads to some of the solutions. Usually, they are in a field book, electronic data collector, or some form of COGO software file on a computer in the surveyor's office. Most often, previously calculated coordinates are held, or new coordinates are calculated, for existing points.

  a surveyor at work
A surveyor measures angles and distances with a theodolite, recording the measurements automatically with a data collector.
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This image shows the improvement of building footprint locations in ArcGIS using traditional surveying techniques.

This process contains a number of problems. First, if the original calculated coordinates are determined to be correct, valuable measurements that can improve geospatial data accuracy are lost. Or possibly new coordinates are calculated to represent existing feature location, and multiple coordinates will exist for a given feature.

These problems are not as difficult to address as it may appear, but it requires a change in how we look at measurements. With current database technology, GIS professionals can treat a measurement as an entry in a database. The measurement will be stored and archived in the database with all the associated metadata (date observed, weather conditions, field crew, instrument specifications, etc.). One can then use these new measurements with measurements previously collected to improve the positional accuracy of existing points and features. Other technology that makes this possible is the ability to link a feature or group of features to a survey point. As the positional accuracy of that point improves, the locations of the associated features remain coincident with that point. This allows one to select specific areas in which to update measurements and accuracy, while not discarding the valuable information already collected. This will improve the cost efficiency of managing GIS. For the surveyor, this opens up a new market. With collaboration between the GIS operators and surveyors, the surveyor becomes a key player with certain types of GIS data updating remaining the backbone of geospatial data accuracy and helping to further bridge the gap between surveyors and GIS professionals.

For more information, contact Brent A. Jones, Survey/Engineering Industry manager (e-mail: bjones@esri.com; tel.: 909-793-2853, ext. 1-1846), or visit www.esri.com/surveying.

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