CADIS Guide to Ground Control Points, 2003

The collection of ground control points was a multi-phase task. Before the actual collection of points could start, we needed a game plan. We initiated our hunt using the existing work that had been done a few years earlier for the QuickBird analysis. Some of these ground control points would be usable, and others would offer us some insight as to what was visible, and what was not. By first identifying old control points that could be used again in this study, we allowed our eyes to get used to things, such as corners and intersections of sidewalks, that would be easily recognizable and visible from the imagery. In addition, the old control points were also seen as an accurate way to check our data. By periodically using the control points and local HARNs, we could check our accuracy against other known measurements.

The first phase was identification of useable, older points. The second phase started shortly thereafter with the locating of other points within our study area that could also be used. Street corners and major intersections were obvious choices since the roads could often be identified on the map. Most areas in the city also have sidewalks and corners, which are excellent choices as markers. The more rural areas posed a problem simply because roads were not as common, and corners were few and far between. During this phase, the area of interest (or AOI) was broken up into 9 columns, and each person was given a column in which to locate possible collection points. After this work was completed, one student compiled all the data and put all the points into an excel spreadsheet. This spreadsheet was used as a guide for people going out in the field. In addition, she ensured that each of the control points was given a unique name based on the column that the point was in (B through I), the row that the point was in (1 through 8) and an addition letter (a through m, usually) to denote how many points were in that section. After this point, the control points were colleted using these names so that each was individual and meaningful.

The third phase relates to the actual collection of data. Using the HARN that is in the front lawn of Old Main, the CADIS students worked on perfecting their protocol using the Trimble ProXR and a tripod. They worked through basic equipment problems, and came up with a way to create a protocol for how things should be done when they collect points in the field. After discovering that they had been able to obtain high accuracy points for the HARN, they created a data dictionary for collecting points in the field. The data dictionary was then loaded on to the rovers, and the units were then taken into the field to collect readings for the possible control points identified in the lab. In order to get consistently good readings, measurements for one point were taken until the counter reached 1500 points. The data dictionary also required an address for the control point as well as a description of the area. After collecting the points, they were loaded onto a computer using GPS Pathfinder Office and were corrected using CAST’s base station. Including delays for weather and other logistical issues, this phase took close to a month.

The final product was to be a document for the City of Fayetteville that included a location map, a description of the point, and the point’s location. This final document was created in Microsoft word and was then adapted into html for use with Internet Explorer. The document consists of an index page complete with a map and links for all of the control points. Each page has the name of the point, a map indicating its location, a description of its location, and the latitude, longitude and height of each point.