Survey Methods for Mapping Detail. Effects of Changing Technology

Survey Methods for Mapping Detail. By its nature and assumptions, plane surveying has been used mainly for smaller areas, especially for infilling detailed mapping within a national control framework. It involves the measurement of distances and angles in order to determine the positions and elevations of points.

The two main approaches to such survey measurement are field techniques on the ground and aerial techniques (photogrammetry).

Although other instruments have been employed, the most common instruments for precise ground survey have been steel tape, theodolite, and tilting level. The theodolite is designed to measure horizontal and vertical angles, and the tilting level is designed to determine the differences in elevation between points. Each is mounted on a portable tripod for field use.

Methods of locating topographic detail by ground survey have been improved with the development of the "total" station, a theodolite-like instrument consisting of three components—for electronic distance.measure- ment, for electronic angle measurement, and for data processing. It is employed primarily for detailed mapping purposes, either using radiation techniques from one location or tacheometry, where detail is located during a survey traverse. Contour lines can also be located with such equipment.

Photogrammetry is the technique and technology of acquiring measurements from aerial photographs that have been acquired in a special overlapping sequence to permit the accurate determination of relative heights as well as horizontal location.

With the development of special cameras and stereoplotting instruments it became so effective for large areas that it was widely adopted by national governments for their topographical mapping programs. Photogrammetric methods have continued to develop, but the most significant recent change has been to digital (or soft copy) photogrammetry.

Despite the success of photogrammetry, ground survey methods can still be more appropriate for detailed large-scale mapping projects and important for establishing ground control points for photogrammetric mapping.

Effects of Changing Technology. Although the principles behind survey accuracy have been maintained, the instruments and techniques, as indicated, are changing. Today satellite methods (specifically, global positioning systems—GPS) have greatly increased in importance for the rapid and accurate provision of control and even for the detailed revision of topographic maps. For example, the Ordnance Survey of Great Britain has fully adopted GPS for all national coordinate systems, with the old survey monuments now rendered redundant.

Although this article has concentrated on metric aspects, other benefits are provided by remote sensing. Aerial photography, developed for intelligence gathering during war, was subsequently employed to improve the mapping and ongoing monitoring of themes such as geology, vegetation, and land use. Infrared photography and sidelooking radar have added to the airborne techniques, and, of course, satellite imaging systems such as multispectral scanners, electro-optical, and synthetic aperture radar are also providing a wealth of new spatial data for environmental mapping.

Indeed, in some situations and with appropriate spatial rectification (the removal of scale variations and image displacements from aerial photography), the new generation of highresolution (submeter) scanners, such as on the Ikonos and Quick Bird satellites, can provide both interpretative and metric data. They can thus provide effective maplike substitutes to support many spatial tasks.

Maps and Spatial Data. Maps are defined as symbolized representations, constructed to reduced scale, of selected features and characteristics, normally of the earth's surface. Unlike photographic or satellite images that record everything that can be detected by a photographic emulsion or scanning sensor, they are always abstractions of reality, designed to improve understanding. Before the arrival of digital computing, topographic maps and plans, plotted directly from field measurements, were the sole sources for all subsequent spatial data and derived maps.

Basic scales for the mapping of Great Britain were allocated according to the nature of the landscape and settlement density: 1:1,250 for urban areas, 1:2,500 for agricultural land, and 1:10,000 for the remaining moorland and mountains. Maps at smaller scales (e.g., at 1:50,000) were normally derived from these.

Since the introduction of computing, fundamental changes have taken place. Existing topographic maps are converted into digital form for storage in computer databases rather than on paper, and spatial data captured by ground survey, photogrammetry or remote sensing can also be transferred directly to these databases. Although maps can still be created by accessing these sources, the data are also available for direct use in other spatial tasks.

For example, 80 percent of the business turnover at the British Ordnance Survey is for electronic data rather than for the sale of maps. The special value of digital over paper maps is not only an accurate locational record of the features of the physical and cultural landscape (topography), but also the linkage of these to a multitude of other thematic data. Both environmental (e.g., vegetation) and statistical (e.g., population census) data are related to natural or administrative boundaries in the landscape.

Databases enriched in this manner offer substantial added value for a range of users through specialized maps or through the data themselves. Geographic Information System software, the established facility for handling such data, is now in use throughout the world. Digital maps can be transmitted electronically, examined on computer screens (desktop or handheld), accessed via the World Wide Web, and need be printed to paper only when required.

Not only have these new possibilities increased the potential of maps and spatial data, but also they have released maps from their traditional paper restrictions and rendered them more versatile and accessible to serve a much wider audience and to support many more tasks.