Section 3: Introduction to Surveying
Anchor: #i1001918Overview
The information found within this section is a product of the former Texas Department of Transportation (TxDOT) Metric Surveying Subcommittee and the Standing Committee on Surveying (SCOS)
Anchor: #i1001928Types of Surveys
Surveying is the science of determining relative positions of points on or near the earth’s surface. The horizontal positions of these points are computed from distances and directions, and vertical positions from differences in elevations which are measured individually or in combination to a specified degree of accuracy, and are in direct relation to a known or determined datum.
Accuracy is a prime consideration in surveying. Instruments for each type of survey are used with prescribed techniques to achieve a designated accuracy.
Less accuracy than specified results in a survey which will prove useless for its intended purpose; more accuracy wastes time and effort, and may not improve the final results.
Surveys are normally divided into two general classes: Geodetic and Plane.
Anchor: #i1001953Geodetic Surveys
The mathematical shape of the earth is an oblate spheroid (almost a sphere) with a major diameter at the equator of about 7,920 miles. Distances or areas measured on the surface of the earth are, therefore, not along straight lines or planes, but are on a curved surface. Geodetic surveys that normally extend over long distances and cover large areas must have computations to allow for curvature of the earth.
To accomplish this, the earth’s major and minor diameters are computed accurately, and from these a spheroid reference. The position of each geodetic station is related to this spheroid. The positions are expressed as latitudes (angles north or south of the equator) and longitudes (angles east and west of the prime meridian), or as plane coordinates on a rectangular grid system, correlated with the latitude and longitude. In addition, the plumb line deflection and its effect on relative positions of the stations are considered in precision work.
Anchor: #i1001968Plane Surveys
When the extent of the survey becomes small (less than 100 square miles in area), and when only limited accuracy is required, the effect of curvature can be ignored. These surveys are treated as if the measurements were made on a plane and are known as plane surveys.
The difference between plane and geodetic surveying can be expressed in terms of plumb lines. In plane surveys, plumb lines are considered parallel, while in geodetic surveys convergence is taken into account.
Highway and railroad surveys, which may extend for hundreds of miles, are usually in a narrow strip and are considered plane surveys. However, a limited computation for the earth’s curvature is necessary in this case. On a long traverse survey, an astronomic azimuth is determined at intervals of several miles.
The astronomic azimuth establishes an astronomic north-south line and may be used to obtain the true direction of a survey line. The azimuth values of the lines between astronomic azimuth stations consider the convergence of the meridians. The methods, operations, and measurements in either type of survey are similar; but since the distances between stations are usually much greater in geodetic surveying, more precise equipment and procedures are used.
Anchor: #i1001993Surveying Field Work
Field work in surveying consists of making and recording measurements. The operations are as follows:
- Measuring distances and angles
to:
- establish points and lines of reference for locating details such as boundary lines, roads, buildings, fences, rivers, bridges, and other existing features
- stake out or locate roads, buildings, utilities, and other construction projects
- establish lines parallel or at right angles to other lines, measure inaccessible distances as across rivers, extend straight lines beyond obstacles such as buildings and do any work that may require use of geometric or trigonometric principles.
- Measuring differences in elevations and determining elevations
to:
- establish permanent points of known elevation (bench marks)
- determine elevations of terrain along a selected line or area for plotting profiles and computing grade lines
- stake out grades, cuts, and fills for construction projects.
- Making topographic surveys wherein horizontal and vertical measurements are combined.
- Recording field notes to provide a permanent record of the field work.
Factors Affecting Field Work
The surveyor in the field must constantly be alert to the different conditions he or she encounters and the requirements of the survey. The weather, terrain, personnel, purpose, and accuracy of the survey, systematic procedures, and the expected rate of progress are some of the factors that will affect the work.
Physical factors such as terrain and weather will affect each field survey in varying degrees. Measurements using telescopes can be stopped by fog, mist, or dust. Swamps and flood plains under high water can impede taping surveys. Lengths of light-wave distance measurements are reduced in bright sunlight. Generally, reconnaissance will predetermine the conditions and alert the survey party to the best method to use and the rate of progress to be expected.
The status of training of the personnel is another factor that affects field work. Experience in handling the survey instruments and equipment can shorten survey time without introducing errors, which would require resurvey. The personnel factor is a variable that will affect the rate of progress.
The purpose of the survey will determine the needed accuracy, which, in turn, will influence the selection of instruments and procedures. For instance, comparatively rough procedures can be used in measuring for earth-moving, but grade and alignment of a highway must be much more precise, and require more accurate measurements. Each increase in precision also increases the time required to make the measurement, since greater care and more observations must be taken.
Each survey measurement will be in error to the extent that no measurement is ever exact. Besides errors, survey measurements are susceptible to mistakes or blunders. These arise from misunderstanding the problem, poor judgment, confusion on the part of the surveyor, or simply from an oversight. By working out a systematic procedure, the surveyor will often detect a mistake when some operation seems out of place.
Survey speed is not the result of hurrying; it is the result of saving time through the following:
- the skill of the surveyor in handling his field equipment
- the intelligent planning and preparation of the work
- the process of making only those measurements that are consistent with the accuracy requirements.
Field Notes
The field notes of the surveyor must contain a complete record of all measurements made during the survey with sketches and narration, where necessary, to clarify the notes. The best field survey is of little value if the notes are not complete and clear. They are the only record that is left after the field party leaves the survey site.
All field notes should be lettered legibly. Numerals and decimal points should be legible and permit only one interpretation. Notes must be kept in the regular field notebook and not on scraps of paper for later transcription. The field notebook is a permanently bound book (not loose-leaf) for recording measurements made in the field.
Field note recording takes three general forms: tabulations, sketches, and descriptions. Two, or even all three forms, are combined when necessary to make a complete record.
Tabulation — Measurements may be recorded manually in a field book or they may be recorded electronically through a data collector. Electronic data collection has the advantage of eliminating reading and recording errors.
Sketches — Sketches add much to clarify electronic data collection files and should be used as a supplemental record of the survey. They may be drawn to an approximate scale, or important details may be exaggerated for clarity. Measurements may be placed directly onto the sketch or keyed in some way to the tabular data. A very important requirement of a sketch is legibility. It should be drawn clearly and large enough to be understandable.
Descriptions — Tabulations with or without added sketches can also be supplemented with descriptions. The description may only be one or two words to clarify the recorded measurements, or it may be quite lengthy in order to cover and record pertinent details of the survey.
Note: Erasures are not permitted in field notebooks.
Individual numbers or lines recorded incorrectly shall be lined out and the correct values added. Pages that are to be rejected are crossed out neatly and referenced to the substituted page. This procedure is mandatory since the field notebook is the book of record and it is often used as legal evidence.
Anchor: #i1002149Electronic Data
In nearly all cases, field work is automated by the use of computer software and hardware for collecting, reviewing and editing field measurements. A data collector is connected to the instrument (total station, GPS receiver, digital level, etc.) to store the raw measurement data and perform coordinate geometry (COGO) functions while in the field. Original raw data must be saved as a file for retention as matter of record before any data editing is done.
Anchor: #i1002159Data Collection
It is recommended that field data in electronic form be collected in the AASHTOWare®, Survey Data Management System ® (SDMS) Collector and processed in AASHTOWare ® SDMS ® Processor. This is software developed by AASHTO and supported by TSD. It is provided gratis to TxDOT consultants under TxDOT’s license agreement with AASHTO. Its purpose is to provide a more flexible and user definable method of recording horizontal angles, vertical angles, and slope distances from total stations in a standard format, for use with survey measurement post-processing software.
Radial topographic survey data, traverses, and level runs may all be collected in SDMS ® Collector software. The data can then be reduced to coordinates using SDMS ® Processor, which uses a least squares type of adjustment. There are a number of useful reports that can be generated in this software.
There are numerous ways to provide connectivity between survey data points. When performing radial topography surveys for a Digital Terrain Model (DTM), points in the same chain such as edge of pavement, centerlines, and ditch lines can be linked together. These survey chains can ultimately be exported to mapping files (2D) or to DTM files (3D) as breaklines. The survey points and breaklines will be used by topographic mapping software to create a Triangular Irregular Network (TIN) and subsequently a DTM. Parcel boundary corners may also be connected with survey chains.
Standard TxDOT feature codes and cells have been developed for use in the field to insure standardization of line weight, color, levels, and symbology. These feature codes also determine whether the points and chains will be exported to a mapping file or a DTM file.
Anchor: #i1002187Survey Review and DTM
Three software programs are authorized by TxDOT to view survey results for troubleshooting and preparation for the delivery of a .dgn file. These programs are Autodesk ® CAiCE TM Visual ® Transportation, Bentley ® GEOPAK Survey TM, and AASHTOWare ® SDMS ® Processor. This software will accept the SDMS .cal or .pac files as input and, with the TxDOT feature table attached, will graphically display the project for analysis. Corrections and additions can be made and the DTM can then be created. Photogrammetry files, background maps, macros for visualization and other enhancements may be utilized before 2D or 3D graphics are exported as a DGN file for GEOPAK ® / Microstation ® use by the designer.