Intro to GPS Apps
Surveying

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Contents

Introduction

The purpose of surveying is to identify the locations of points on or near the earth. Surveys are performed to determine several pieces of information:

Probably the most common survey in the United States is the legal land survey. This survey is required to establish the legal definition of a piece of land which is offered for sale. If you purchase a house in the United States, the parcel of land your house will rest on has probably been surveyed and the results registered in your local county/parish/municipality. As new states were added to the United States, each territory was surveyed so that parcels of land could be sold to settlers with the results going to the public coffers.

Another common type of survey is the engineering survey. Any time a road, dam, bridge or other large facility is built, the land is surveyed so that the engineers can determine how the facility will interact with the terrain. In many cases, large quantities of dirt or other material must be added to or removed from a construction site. The survey results are used to determine how much material must be moved, which is then included in the calculations of the cost of the project.

Advantages of GPS Surveying

For hundreds of years, surveyors have relied on optical instruments and physical measuring devices (such as tape measures or chains). Optical instruments (and newer electronic distance measuring (EDM) instruments) require direct line of site from the instrument to a target. Measuring tapes or chains require that the survey crew physically pass through all the intervening terrain to measure the distance between two points. The big advantage of GPS is that line of site does not have to be established between two stations. Thus, surveying can be done in almost all weather conditions or on opposite sides of a mountain. Another advantage is that the accuracy of the collected data is not as dependent on the skill of the instrument operator.

Because line of sight does not have to be established between GPS stations, major cost savings can be realized in large projects involving a large number of survey teams over a limited area (say, 100 sq miles). A single GPS receiver can be set up as a reference station which can be used by any number of surveyors, each of which can be working a job. This contrasts with conventional survey equipment in which at least two people must be working the same job (one for each end).

Accuracy Requirements

Control Surveys

Control surveys are used to establish the locations of arbitrary points. These points, called control points, may then be used as reference locations for performing additional survey work. Often, the reason for performing a control survey is to place control points in locations which are physically convenient for the intended survey work. Control surveys are generally performed to a higher standard of accuracy than other types of surveys. This is necessary because any follow-on survey work must be able to count on the accuracy of the control points.

Control surveys usually consist of several different measurements to the unknown point from several known points, so that the measurements are taken from different angles. The measurements are combined in a process known as network adjustment, in which existing known points are held fixed, and the position of unknown point adjusted until it best fits the measurements. The accuracy of the network adjustment is dependent on several factors:

Radial Surveys

Radial surveys are surveys in which the arbitrary point to be located is connected to the control network at only one place. It is assumed that the accuracy of the survey technique is sufficient to locate the point with acceptible accuracy. This type of survey is used for general mapping.

See Also...

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