differentialGPS singlePositionGPS photogrammetry terrestrial approximate fromMap
A number of different methods may be used to determine position. They are categorized into six basic choices.
differentialGPS: Is the position of a point with repect the GPS satellite network and a second point where the second point is taken as the origin of a local coordinate system. Several simultaneous GPS readings for the related points are determined. These multiple redundant readings are used to correct for atmopheric propogation delay, receiver error and error in satellite epherides. This is also known as relative positioning and has an associated error of 0.1 to 2.0 ppm. (Guide to GPS Positioning, 1986)
singlePositionGPS: Is the position of a single stationary or moving point or object derived through the use of a single GPS unit. This location is determined with respect to a well defined coordinate system that is itself oriented with respect to the earth. If three satellites are visible to the antenna at a point accuracy down to decimetres should be achieved. (Guide to GPS Positioning, 1986)
photogrammetry: Positioning based on generally accepted photogrammetric methodology. The position derived is based on the fact that a definite geometric relationship exist between the relative spatial positions of image points in a two dimensional photograph and their corresponding positions in 3 dimensional object space. In general the accuracy of photogrammetric positioning is related to the scale of the associated photography. (Manual of Photogrammetry, 1980)
terrestrial: This includes traditional survey networks as well as open and closed traverses where position is derived using high quality equipment and defined procedures. Accuracy will be dependent on specific equipment and methodology.
approximate: An approximate location would include field procedures such as pacing, tape and compass, compass and clinometer.
fromMap: This refers to positions that are derived from a published topographic map. Associated accuracy will depend on methods of measurement and on the scale of the map used.