Page 71 - Green - Maritime Archaeology: A Technical Handbook. 2nd ed

50 Maritime Archaeology: A Technical Handbook, Second Edition
areas close to the poles. The Mercator projection exaggerates the distance between meridians by the same degree as the lengths of the parallels in order to obtain an orthomorphic projection. A transverse Mercator is similar, but based on the transverse cylindrical projection. One needs to know how these projections affect the measurement of distance. For example, in the Univer- sal Transverse Mercator (UTM) projections only work up to about 5° of lon- gitude before they become inaccurate. So it is essential to check the limitations of any projection system used. In general, the larger the scale that one is working in the more complicated the problems.
5. Coordinate Systems
There are two basic and commonly known coordinate systems used with the GPS: a Cartesian coordinate system and the UTM. The former has its origin near the geographical center of the Earth, the Z-axis parallel to the terrestrial poles and the X-axis passes through the Equator at the inter- section with the prime meridian. This system uses the World Geodetic System 1984 (WGS84) datum reference ellipsoid and usually uses degrees, minutes, and seconds; degrees and decimal minutes; or decimal degrees as units for latitude and longitude and a distance for altitude. The WGS84 system has universal application and is widely used in marine surveying. The UTM is a projection system which has different ellipsoids for different parts of the world and uses a linear measurement of easting and northing (usually in meters). The UTM is usually used in land survey work, but it has an especially useful application in marine work because of the ability to set lane spacing in north–south, or east–west directions to precise metric values. Because UTM is a projection, the datum it uses can be WGS84 (usually the choice for marine work) or other datum depending upon where you are and the local convention.
Because most GPSs have the ability to translate information from one system to another quite easily, there are often advantages in moving from one system to another. The most likely problem is that a GPS usually down- loads position data in WGS84 in decimal degree format, so that basic survey data will need to be translated into UTM using some sort of software package.
III. VISUAL SEARCH TECHNIQUES A. INTRODUCTION
Having determined a way of locating a position at sea, methods of locat- ing sites now need to be considered. It is surprising how frequently one