Journal of Surveying Engineering

GPS computations are performed in the geocentric coordinate system and transformed to geographic (latitude/longitude) or state plane coordinates for extended use in data‐base applications or engineering projects. State plane coordinates are very useful, but for some applications the difference between grid and ground distance becomes particularly bothersome. State department of transportation (DOT) offices are confronted with the grid/ground distance dilemma on highway projects, for which center‐line stationing must be ground based and state plane coordinates are used for project control. A questionnaire was sent to all 50 state DOTs asking for input as to how the problem is currently being addressed and asking for suggestions for solutions. Responses received from 46 of the 50 state DOTs are summarized in an appendix. The proposal included in this paper is to use the geocentric coordinate system as the basis for all three‐dimensional control computations, including conventional terrestrial measurements. To eliminate the distance distortion problem, results can be outputted in a local coordinate system. Equations for transforming between geographic, geocentric, and local coordinate systems are included in an appendix.

The Ashtech 3‐dimensional Direction Finding system (3DF) utilizes carrier phase measurements from the Global Positioning System (GPS) to provide real‐time attitude information along with highly accurate position and velocity. The 3DF system includes a 24‐channel receiver and four microstrip antennae. The receiver's 24 channels are configured in four six‐channel banks. Each bank is connected to a separate antenna. Attitude is the angular orientation of a body with respect to some reference. This orientation is determined using the GPS signals and knowledge of the precise relative antenna positions. Mounting at least three antennae on a rigid platform and determining the relative positions provides three‐dimensional attitude information. The Ashtech 3DF system uses four antennae for rapid determination of differential phase ambiguities and for continuation of position and attitude information even when satellite visibility is low. Potential applications of the 3DF system include land, marine, and airborne navigation. Specific issues that arise in these applications are discussed in detail. Test results of the 3DF in several environments will also be presented.

The use of global positioning systems (GPS) as a survey tool can bring rapid solutions for transportation management. GPS can be an efficient cost‐effective, and accurate method of obtaining positional data, as reported by Henderson and Quirion in 1988. Field conditions can limit the use of GPS. To use GPS effectively requires innovation, expertise, and experience on the part of surveyors, as noted by Craymer et al. in 1990. GPS data combined with a total station data were used to provide control along a 27.5 km (17.1 mi) section of the Santa Monica Freeway, in California. A total of 54 primary stations were positioned with GPS and 203 secondary stations were positioned with a total station. The static method was used in lieu of the kinematic method to position primary stations. For reasons explained in the paper, it would have been difficult if not impossible to maintain lock on the satellites during the progress of the survey and collect valid data for positional computation, as reported in Coad, in 1988. The survey control was completed in order to digitize the traveled edges of the freeway. The digitizing and compilation of maps was completed directly from the photos and not by a stereo compilation.