Chapter 3: Preliminary Surveying

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Section 1: Horizontal Control Surveys

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Overview

Information contained in this section is excerpted in its entirety and/or adapted for this manual from the Texas Society of Professional Surveyors Manual of Practice.

A horizontal control survey is performed for the purpose of placing geographic coordinates of latitude and longitude on permanent monuments for referencing lower levels of surveys. A projection is used to place the coordinates on a plane of northing and easting values for simplified measurements. Scale and elevation factors are applied to make the distance measurements applicable to the exact project location on the working surface and the type of projection chosen is an “equal angle” type.

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Justification

The use of assumed coordinates for a new project is not acceptable. Placing all projects on a common coordinate system is more cost effective for the long term. The same control points can be used at a later date on subsequent surveys; a tight network of points can be developed across the area.

Consultants can be instructed to use only these designated points, eliminating the possibility of tying to faulty existing control. With proper use of surface adjustments from the state plane grid, continuity will be maintained from one area to the next, allowing easier detection of blunders and systematic errors. Lost monuments can be replaced to their original location. Work by other agencies, including useful maps and Geographic Information Systems (GIS) data, can be overlaid adding to the overall usefulness and completeness of everyone’s efforts.

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Resources

Specifications and procedures must adhere to the Texas Society of Professional Surveyors (TSPS) Manual of Practice, Category 7. Federal publications, which define the basic specifications and procedures, include:

Horizontal Control Data Technical Report NOS '88 NGS 19

Manual of Plane Coordinate Systems Special Publication No. 235

Manual of Traverse Computations on the Lambert Grid Special Publication No. 194

Plane Coordinate Projection Tables “Texas” (Lambert) Special Publication No. 252

Standards and Specifications for Geodetic Control Networks by Federal Geodetic Control Committee, 1984

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Field Methods

Particularly for horizontal control surveys, GPS is quickly replacing the use of the total station for long distance traversing. The inherent error of each GPS derived baseline (about 5 mm plus 1 part per 1,000,000) will make accuracy at short distances not so attractive. However, using baselines of many miles suddenly becomes phenomenally accurate and cost effective. The distant locations of Continuously Operating Reference Stations (CORS) stations and only scattered high order monuments are not a problem.

A conventional traverse, if using state plane coordinates as the point of beginning (POB), will require every horizontal distance of each leg of the traverse to be multiplied times the CAF of the midpoint of the leg in order to “traverse on the grid.” If this is not done, the resulting point will not be positioned correctly on the state plane grid. A technical description of the process, including application of Second Term correction can be found in Chapter 4 of the NOAA Manual NOS NGS 5, State Plane Coordinate System of 1983.

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GPS Control

To meet a network or local accuracy level, a GPS project must be connected to sufficiently accurate and well-distributed existing control.

All of the control stations to which the network will be constrained must have positions known on the NAD83 datum. Control of the NGS CORS Network are generally used; however, certain special projects may have a legitimate need for another geodetic reference. Use the appropriate datum adjustment as recommended by the TxDOT district surveyor or survey coordinator.

The minimum number of horizontal and vertical constraints is stated in Table 3.11 Minimum TxDOT Network Design Specifications in this chapter with their location being distributed in different quadrants relative to the center of the project. Where existing NGS or TxDOT horizontal and/or vertical control on a common datum and epoch is available, all such stations lying within a few kilometers of the survey’s boundaries should, if possible, be included in the survey if they meet the horizontal accuracy requirements. Second order or better is generally required for vertical.

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Rights of Access

As with any survey, permission to enter property must be obtained and arrangements must be made with property owners, tenants, or agents responsible for the property. Depending upon the area, hours of work, and nature of traffic control, local law enforcement officers may need to be notified.

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Monuments

Because of the expense and time involved in the accurate determination of position of horizontal monuments, the setting should be permanent and in a location suitable for GPS observation.

The pre-punched, embossed disk for horizontal control is DHT # 164946 and is used for mounting in rock or concrete.

The same face design can be found as a rebar cap with DHT # 164949.

The preferred setting; however, is a rod driven to refusal and protected by a PVC encasement with a flip-lid cover.

Appendix C, “ Monumentation,” lays out specifications for the three above mentioned methods of monumenting a control point. Whether an aluminum disk or datum point rod is used, the point name should be legibly stamped into the metal surface on the face of the disk, or into the machined surface on the rim of the cover.

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Monumentation for New Stations

All monumentation for new Level 1 points are to be in accordance with the following NGS publications:

Concrete Marks, from NGS Operations Handbook and Manual of Geodetic Triangulation, S.P. 247.

Setting a Survey Disk in Bedrock or a Structure from NOAA Manual NOS, NGS 1, Geodetic Bench Marks.

Setting a NGS 3-D Monument Based on Revised NGS 3-Dimensional (3-D) Rod Mark [Draft Version] by: Curtis L. Smith, National Geodetic Survey, July, 1996.

It is recommended that new Level 2 points also follow these construction specifications or they may follow the specifications for the TxDOT datum point rod or the standard poured concrete setting (Type II setting) as found in the TxDOT Survey Manual.

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Azimuth Marks

The use of radio masts, church steeples, etc. for azimuth marks is not as practical as it once was. The coordinates that have been used for these intersection stations for years may not be on the same datum or datum adjustment as the newly set monument. As a safeguard, a monument should be placed within about a quarter or half mile of the station and surveyed at the same time. Occasionally, it will be possible to set project control points inter-visible but the terrain may not permit this convenience.

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Tolerances

The following tables show tolerances for various conditions for conventional horizontal surveys and for GPS horizontal control surveys. Note that with the use of GPS, there are fewer physical checks and there is more attention paid to the number, location, and quality of reference monuments due to the nature of GPS static surveying.

The following table describes the Texas Society of Professional Surveyors (TSPS) Manual of Practice tolerances for conventional traverse:

Anchor: #i1021130Table 3.1 TSPS Tolerances for Conventional Traverse (Category 7)

Condition

I

II

III

 

 

TSPS 1st Order

TSPS 2nd Order

TSPS 3rd Order

Remarks

Error of Closure

1: 50,000

1:20,000

1:10,000

Loop or between monuments

Allowable Angular Closure

± 3''

± 8''

± 15''

N= number of angles in traverse

* Accuracy of Bearing in Relation to Course

± 04''

± 10''

± 20''

Maximum for any course

Linear Distance Accurate to: (Minimum Length of line)

1: 50,000

(2500 feet)

1: 20,000

(1000 feet)

1: 10,000

(500 feet)

 

Positional Tolerance of Any Monument

AC/ 50,000

AC/20,000

AC/10,000

AC = length of any course in traverse

Scale of Maps Sufficient to Show Detail but No Less Than:

1'' = 5,000'

1'' = 2,000'

1'' = 1,000'

 

Positional Error in Map Plotting not to Exceed: (applies to original map only)

125 ft.

50 ft.

25 ft.

Generally 1/40”

(National Map accuracy calls for 1/50”)

Adjusted Mathematical Closure of Survey No Less Than

1:200,000

1:200,000

1:200,000

 



NOTE: *All bearings or angles shall be certified as based on one of the following sources: 1.) Geodetic Bearing, 2.) Grid Bearing of the Texas Coordinate System of 1927 (or 1983), (with the proper zone identified) or 3.) a record bearing or the relation thereto, along a line monumented on the ground as called for in said record.

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Accuracy Standards

The new standards support both local and network accuracies:

  • The local or relative accuracy of a control point is a value that represents the uncertainty in the coordinates of the control point relative to the coordinates of other directly connected, adjacent control points on the project at the 95-percent (2σ) confidence level.
    • The reported local accuracy is an approximate average of the individual local accuracy values between a control point and other observed control points used to establish the coordinates of the control point (i.e. the adjacent stations directly tied to the control point).
  • The geodetic accuracy of a control point is a value that represents the uncertainty in the coordinates of the control point with respect to the geodetic datum at the 95-percent confidence level.
  • For National Spatial Reference System (NSRS) network accuracy classification, the datum is considered to be best expressed by the geodetic values at the Continuously Operating Reference Stations (CORS) supported by NGS. By this definition, the local and network accuracy values at CORS sites are considered to be infinitesimal, i.e., to approach zero.

Local or relative accuracy is best adapted to check relations between nearby control points; for example, a surveyor checking closure between two NSRS points is most interested in a local accuracy measure.

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TxDOT Standard Levels of GPS Accuracy

TxDOT has numerous survey accuracy requirements based on the type of project being surveyed. Listed in the table below are seven levels with typical types of surveys for each. The level of survey accuracy will be used as a standard throughout this manual to define the quality of the survey measurements for a particular application.

The seven levels, of course, are of equal or lesser accuracy than the A and B order federal monuments that could be considered Level 0 in the TxDOT scheme. A Level 0 is shown in some of the charts only to show the relationship of the NGS points to the seven TxDOT levels.

Anchor: #i1008804Table 3.2 TxDOT Level of Survey Accuracy

TxDOT Level of Accuracy

Typical Applications

Level 0

CORS, FBN, CBN (this level overseen by NGS)

Level 1

Statewide/district-wide Control Densification, RRP Network Stations (until recognized as NGS CORS), Cooperative CORS sites

Level 2

Primary Project Control, Control for Airborne GPS for Photogrammetry or LiDAR Data Gathering

Level 3

Photogrammetric Control Panels, Boundary Corners, ROW, and Local Control

Level 4

Topography, Stakeout

Level 5

Sub-meter Mapping for GIS (includes inventory and locative surveys)

Level 6

1 – 5 Meter Mapping for GIS (includes inventory and locative surveys)

Level 7

> 5 Meter Mapping for GIS (includes inventory and locative surveys)



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The End Product

It must be remembered that if the survey is performed using GPS, the basic results should be state plane coordinates. If a grid to surface adjustment is applied to all coordinates across the board at this point, significant differences over even short distances will accumulate. This is a necessary evil when working on the project surface but for control points to be used again and again on different projects requiring different combined adjustment factors (CAF), the final plane coordinate recorded on a control point data sheet must be a state plane coordinate. Each point must be dealt with individually to assign a surface coordinate to the monumented point.

A separate TxDOT Control Point Data Sheet signed and sealed by a Registered Professional Land Surveyor (RPLS) licensed in Texas for each newly set monument (to be used for control) shall be prepared showing the quality of the point and the monuments used to establish the control point, as well as the usual information associated with control points. The control point data sheet is meant to convey all necessary information for anyone to use this monument at any time without any doubts or further research.

The point naming convention may vary from one district to another by the use of prefixes and suffixes but the base numbering should contain three (3) digits designating the county number, followed by four (4) digits to be used as the discrete control point number (which is not to be duplicated in the county). If allowed by the district, it is also possible to number the points using the U.S. National Grid (USNG) numbering scheme.

No matter which method is used, any printed list, file, or map must indicate the appropriate datum (including datum adjustment), the unit of length, and the state plane zone (if plane coordinates). Further, if the points are project specific (have had a surface-grid adjustment applied), the chosen CAF must be included.

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