Global Positioning System (GPS)
Equipments

Land surveying instruments have a long history. Since the concept of boundaries came into existence, then the pyramids of Egypt, when buildings reached massive proportions, land surveying instruments played a critical role in the location and construction of virtually every plot of land was divided, and every building erected ever since.

American history still acknowledges the great surveyors in our history. George Washington, the leader of the Colonial Army and first president started his career as a land surveyor, starting when he was only 16 years old and lasting several years. The accepted demarcation between the southern and northern states is the border surveyed from 1763 to 1767 by Charles Mason and Jeremiah Dixon still known today as the Mason-Dixon line.

Over the years, a number of various instruments have been invented for measuring distance, direction, vertical and horizontal positions, time and astronomical location. Many have been overtaken by technology which has replaced several different instruments with all-in-one total solutions.

There are still some "old school" surveyors still in business, however to be truly competitive – and often to be in compliance with federal, state and local laws, regulations and ordinances – some form of high-tech laser-based, GPS-enabled, calibrated chronometer, and sometimes astronomically-capable device is not only necessary, but required.

Map tables are rarely used anymore, and the alidade has all but disappeared. The last application for this venerable optical device was the US Forest Service which used a specific type of alidade – a transit used in conjunction with a map table – for locating fires. Today, aerial location is generally much faster and more accurate in locating fires.

Today's most modern technology includes a combination of optics - to roughly orient the device, dual selectable lasers, prodigious amounts of memory to record tens of thousands of measurements, and dozens of jobs simultaneously, response times of calculations in less than a half second and accuracy rapidly approaching the parts per billion range. Affordable units can measure accurately to less than 2 parts per million.

There is a distinction between land survey instruments and contractor survey instruments. For the most part, routine home construction simply does not require the degree of accuracy an instrument measuring distances approaching 3 miles. Yet, construction transits and total solution systems have hardly been lagging the long-range high-tech units. Multiple synchronized lasers capable of 3-dimensional full 360 degree measurements can - and do - perform and record a number of operations simultaneously, literally at the push of a button.
Few of the most commonly used instruments for land surveyors are as under :
Total Stations

Global Positioning System (GPS)A total station is an optical instrument used in modern surveying. It is a combination of an electronic theodolite (transit), an electronic distance measuring device (EDM) and software running on an external computer.

With a total station one may determine angles and distances from the instrument to points to be surveyed. With the aid of trigonometry, the angles and distances may be used to calculate the coordinates of actual positions (X, Y, and Z or northing, easting and elevation) of surveyed points, or the position of the instrument from known points, in absolute terms. The data may be downloaded from the theodolite to a computer and application software will generate a map of the surveyed area. Most modern Total Station instruments measure angles by means of electro-optical scanning of extremely precise digital bar-codes etched on rotating glass cylinders or discs within the instrument. The best quality total stations are capable of measuring angles down to 0.5 arc-second. Inexpensive "construction grade" total stations can generally measure angles to 5 or 10 arc-seconds. Measurement of distance is accomplished with a modulated microwave or infrared carrier signal, generated by a small solid-state emitter within the instrument's optical path, and bounced off of the object to be measured. The modulation pattern in the returning signal is read and interpreted by the onboard computer in the total station, and the speed-of-light lag between the outbound and return signal is translated into distance. Most total stations use a purpose-built glass prism as the reflector for the EDM signal, and can measure distances out to a few kilometers, but some instruments are "reflectorless", and can measure distances to any object that is reasonably light in color, out to a few hundred meters. The typical Total Station EDM can measure distances accurate to about 0.1 millimeter or 1/1000 foot, but most land surveying applications only take distance measurements to 1.0 mm or 1/100 foot.
Some modern machines are 'robotic' allowing the operator to control the machine from a distance via remote control. This eliminates the need for an assistant staff member to hold the reflector prism over the point to be measured. The operator holds the reflector him/herself and controls the machine from the observed point.


Global Positioning System (GPS)

Global Positioning System (GPS)The Global Positioning System is a satellite based navigation system developed by the United States Department of Defense. It is widely used for civilian navigation and positioning, surveying and scientific applications.

Position accuracy depends on the receiver's ability to calculate accurately the time it takes for each satellite signal to travel to earth. There are various factors affecting the accuracy of GPS signals. The differential mode of operation, also known as DGPS, is an improvement for better accuracy.

DGPS is generally used at the control points to establish accuracy of stations. The control points established through DGPS can be used to start the topographic survey. The observation at this point is taken for a long period of time, till the time 2DRMS error is within acceptable limits.

GPS in stand alone mode is also used in many projects for capturing various features and details. The accuracy however is limited.

GPS is normally operated in differential mode (DGPS), therefore enhancing the accuracy of measurements and eliminating most of the above mentioned errors.
Land surveying instruments
Land surveying instruments