Journal of Surveying Engineering

An inexpensive and simple technique was developed for the survey of inaccessible objects, in which the image of the target is formed at an accessible location by means of an optical beam splitter. The three dimensions of the inaccessible object are determined from measurements conducted at the location of the object image and observed through the transparent component of the beam splitter. A simple prototype was built for tall vertical structures, horizontal or inclined roofs, and objects at potentially dangerous locations. With this prototype, the image may be formed at a convenient location for conducting the measurements, and cross sections can be determined at different planes. The beam splitter of the prototype is a 50/50 reflectance/transmittance half‐silvered mirror. The technique was used to subdivide a distance between two inaccessible targets, and also to map an ancient dome for restoration purposes. The time saved with the proposed technique in comparison with indirect theodolite surveying is considerable. The accuracy obtained matches the accuracy of optical distance‐measurement techniques, and can be largely increased by using prism‐binocular field glasses.

The accuracy of Global Positioning System (GPS) relative positioning depends on the geometric distribution of the observed satellites and on the quality of the observations. Different types of errors affect GPS relative positioning in different ways. Some of the errors may have systematic effects on the measured baselines, which may produce significant scale errors and rotations. These effects, however, may change from one survey epoch to another due to a change in the error characteristics and/or the sky distribution of the observed satellites. Therefore, they cannot be completely canceled out in the computation of deformations. In monitoring and engineering surveys of high precision, these effects must be modeled and eliminated. This paper discussed the effects and their modeling. As an example, a GPS ground‐subsidence monitoring network in Venezuelan oil fields was analyzed. Rotation of up to 6 ppm in a vertical plane was obtained between survey campaigns in 1990 and 1991. To significantly minimize these systematic effects, some recommendations are made for the design of monitoring schemes and control networks for large engineering projects.

This paper provides a case study of the creation of a parcel‐based land information system for a large metropolitan county. The paper describes the structure of the system and the technical specifications used in its creation; the attendant costs; the institutional structure created to implement the system utilizing a combination of public and private funding sources; and the work accomplished to date toward completion of the system, including work on the required remonumentation of the U.S. Public Land Survey system, on the required horizontal and vertical control surveys, on the large‐scale topographic and cadastral mapping, and certain related activities. The project provides for the creation over time of a decentralized parcel‐based land information system in which individual participants add their own data to a common automated base map and under which that map becomes the medium through which a broad range of data can be readily exchanged between all of the participants.