【正文】 han the length of tape, then we can divide the long distance into several segments and get the total distance by plus each segment together. Since different tapes have different starts of zero of the tapes, it is very important to judge where the zero of the tape begins.3. Tacheometry and StadiaTacheometry is an optical solution to the measurement of distance. The word is derived form the Greek Tas, meaning “swift”, and metrot ,meaning “a measure”. Tacheometry involves the measurement of a related distance parameter either by means of a fixedangle intercept. Theodolite tacheometry is an example of stadia system. The theodolite is directed at the level staff where the staff is held vertically and the line of sight of the telescope is horizontal. By reading the top and bottom stadia hairs on the telescope view and then the horizontal distance form center of instrument to rod can be obtained by multiplying the stadia interval factor K by the stadia interval and plus the distance C which is form the center of instrument to principal pocus, . D=Ks + C. Usually the nominal stadia interval factor K equals 100 which is a constant for a particular instrument as along as conditions remain unchanged, but it may be determined by observation in practice. The value of C is determined by the manufacturer and stated on the inside of the instrument box. For externalfocusing telescopes, under ordinary condition, C may be considered as 1 ft without error of consequence. Internalfocusing telescopes are so constructed that C is 0 or nearly so。 this is an advantage of internalfocusing telescopes for stadia work. Most instruments now used for stadia are equipped with internalfocusing telescopes.Applications of tacheometry include traversing and leveling for the topographic surveys, location of detail surveys, leveling and field pletion surveys for the topographic mapping, and hydrographic mapping. The relative precision is 1:1000 to 1:5000.Stadia is a form of tacheometry that uses a telescopic crosshair configuration to assist in determining distances. A series of rod reading is taken with a theodolite and the resultant intervals are used to determine distances.4. Electronic Distance Measurement(EDM)The Electronic Distance Measurement (EDM) was first introduced in 1950s by the founders of Geodimeter Inc. The advent of EDM instrument has pletely revolutionized all surveying procedures, resulting in a change of emphasis and technique. Distance can now be measured easily, quickly and with great accuracy, regardless of terrain conditions.EDM instruments refer to the distance measurement equipments using light and radio waves. Both light waves and radio waves are electromagnetic. They have identical velocities in a vacuum (or space) to 299,177。, which are affected by the air’s density, are reduced and need to be recalculated in the atmosphere. The basic principle of EDM instruments is that distance equals time multiplied by velocity. Thus if the velocity of a radio or light wave and time required for it to go from one point to another are known, the distance between the two points can be calculated.The EDM instruments may be classified according to the type and wavelength of the electromagnetic energy generate or according to their operational range. EDM instruments use three different wavelength bands:(1) Microwave systems with rang up to 150 km, wavelength 3cm, not limited to line of sight and unaffected by visibility。(2) Light wave systems with rang up to 5 km (for small machines), visible light, lasers and distance reduced by visibility。(3) Infrared systems with rang up to 3 km, limited to line of sight and limited by rain, fog, other airborne particles.Although there is a wide variety of EDM instruments available with different wavelengths, there are basically only two methods of measurement employed which may divide the instruments into two classification as electrooptical (light waves) and microwaves (radio waves) instruments. These two basic methods are namely the pulse method and more popular phase different method. They function by sending light waves or microwaves along the path to be measured and measuring the time difference between transmitted and received signals, or in measuring the phase differences between transmitted and received signals in returning the reflecting light wave to source.Modern EDM instruments are fully automatic to such an extent that, after the instruments, set up on one station, emits a modulated light beam to a passive reflector set up on the other end of the line to be measured. The operator need only depress a button, and the slope distance is automatically displayed. More plete EDM instruments also have the capability of measuring horizontal and vertical or zenith angles as well as the slope distance. These instruments referred to as total station instruments.5. TraversingA survey traverse is a sequence of lengths and directions of lines between points on the earth, obtained by or from field angle and distance measurements and used in determining positions of the points. The angles are measured using transits, theodolites, or total stations, whereas the distances can be measured using steel tapes or EDM instruments. A survey traverse may determine the relative positions of the points that it connects in series, and if tied to control stations based on some coordinate system, the positions may be referred to that system. From these puted relative positions, additional data can be measured for layout of new features, such as buildings and roads.Since the advent of EDM equipment, traversing has emerged as the most popular method to establish control networks such as basic area control, mapping, control of hydrographic surveys and construction projects. In engineering surveying, it is ideal way to surveys and dimensional control of routetype projects such as highway, railroad, and pipeline construction. In general, a traverse is always classified as either an open t