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Section 2: Vertical 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 TSPS Manual of Practice.

A vertical control survey is performed for accurately determining the orthometric height (elevation) of permanent monuments to be used as bench marks for lower quality leveling. Spirit leveling is the usual method of carrying elevations across the country from “sea level” tidal gauges. However, Global Positioning System (GPS) can be used indirectly but with less accuracy. Height measurements from the ellipsoid (as opposed to the “sea level” geoid) can be determined very accurately with GPS and only with GPS. Trigonometric leveling, with a total station, is not acceptable for vertical control work.

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Justification

Due to the federal government’s placement of quality bench marks across the county, we have always enjoyed the advantage of having a common vertical datum. With technology as it is today, we can more easily extend elevations to remote areas and minor construction projects. Digital levels and GPS make it practically inexcusable to not have seamless project boundaries. Although construction may only require relative heights (an assumed datum), bringing proper vertical to project areas is advantageous in that bench marks can be used for adjoining projects, saving costs, and errors can more easily be detected having more points in common to check against.

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Resources

Specifications and procedures must adhere to the TSPS Manual of Practice, Category 8.

Federal publications that also define procedures and specifications include: USC&GS Special Publication 239 Manual of Geodetic Leveling; USC&GS Special Publication 240; Manual of Leveling Computations and Adjustments; NOAA Manual NGS 3 Geodetic Leveling; and Control Leveling NOAA Technical Report NOA 73 NGS 8.

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Field Methods

The use of first order leveling is cost prohibitive and unnecessary in most cases for TxDOT operations. Discrepancies between originally run level lines in some cases negate the advantages of the precision of the first order and sometimes second order level runs. The instrument should be treated with care and a peg test should be done often. Level rods are equally critical. Shots should be balanced.

Most digital levels have on-board adjustment programs and/or a memory card that will allow the data to be transferred to a computer for adjustment. The Survey Data Management System® (SDMS) collector, at this point is developed to the extent that it can communicate directly with a level to automatically pull down the information. Manual readings can also be hand entered into the data collector to record the data, warn of out-of-tolerance readings, adjust the point elevations, and compile reports.

A carefully planned GPS network survey can be used to obtain orthometric heights. Since GPS measures heights from the imaginary ellipsoid surface, the data must be converted to usable orthometric heights through a model of interpolated geoid separation measurements.

In order to get the accuracy needed for a vertical control survey, there must be at least 3 or 4 high quality bench marks surrounding the project area (and in 3 or 4 separate quadrants) to better model the area.

This sometimes makes the use of GPS impractical – spirit leveling may be just as cost effective, if the distances are not too great. See Section 7, GPS Static Surveying, and Table 3.11 Minimum TxDOT Network Design Specifications, in this chapter for specific instructions on performing vertical surveys with GPS. Also, see the subsection, Determining Elevations, in this chapter.

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Rights of Access

As with any survey, permission to enter private property must be obtained and arrangements must be made with the property owners, with tenants or parties agents in charge. 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 elevation on a bench mark, the setting should be permanent and in a location suitable for GPS observation. The pre-punched, embossed disk for vertical control is DHT # 164948 for mounting in rock or concrete. The same face design can be found as a rebar cap with DHT # 164947.

However, the preferred setting 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 vertical control point. Whether an aluminum disk or datum point rod is used, the point name should be legibly stamped on the metal surface on the face of the disk, or into the machined surface on the rim of the cover.

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Tolerances

Generally, when a GPS network survey is done, both vertical and horizontal are the end products since processing of data is done using Cartesian coordinates. The tolerances and specifications shown in the tables of the preceding section (Horizontal Control Surveys) include information for the vertical component of the GPS survey.

Table 3.3, available from the TSPS Manual of Practice for Land Surveying in the State of Texas, April 2002, shows tolerances for conventional leveling.

Table 3.3 TSPS Tolerances for Conditions (Category 8)

Condition

I

II

III

 

 

TSPS 1st Order

TSPS 2nd Order

TSPS 3rd Order

Remarks

Error of Closure

4 mm

0.017

8 mm

0.035

12 mm

0.05

Loop or between Control Monuments

Maximum Length of Sight

250 ft.

300 ft.

 

With good Atmospheric Conditions

Difference in Foresight and Backsight Distance

±10 ft.

±20 ft.

±30 ft.

Per Instrument Set Up

Total Difference in Foresight and Backsight Distances

±20 ft. per sec.

±50 ft. per sec.

±70 ft. per sec.

Per Total Section or Loop

Recommended Length of Section or Loop

2.0 mi.

3.0 mi.

4.0 mi.

Maximum Distance Before Closing or in Loop

Maximum Recommended Distance Between Bench Marks

2000 ft.

2500 ft.

3000 ft.

Permanent or Temporary Bench Marks Set or Observed along the Route

Level Rod Reading

± 0.003 M

± 0.001 ft.

± 0.003 M

± 0.001 ft.

± 0.003 M

± 0.001 ft.

 

Recommended Instruments and Leveling Rods

Automatic or Tilting w/Parallel Plate Micrometer Precise Rods

Automatic or Tilting w/ Optical Micrometer Precise Rods

Automatic or Quality Spirit Standard, Quality Rod

When two or more Level rods are used, they should be identically matched

Principal Uses

Broad area control, subsidence or motion studies jig & tool settings

Broad area control, engineer projects basis for subsequent level work

Small area control, drainage studies, some construct and engineer

 



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GPS Orthometric Height

Requirements for orthometric height constraints are dependent upon geoid slope, project extent, desired accuracy, and the density of the gravity database.

In general, vertical control for Level 1 and Level 2 networks require a minimum of 4, preferably 5 published vertical control stations. They should be situated on the outside corners of the project at a minimum.

In other words, at least one bench mark should be fixed in each of the four (4) quadrants of the survey area, such that nearly all of the newly surveyed stations will fall inside a boundary drawn around the outside bench marks. Additional bench marks inside the perimeter will aid in strengthening the adjustment.

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The End Product

The vertical datum in use by TxDOT is the NAVD 88 datum. This datum has replaced the older NGVD 29 datum and shows elevations to be slightly greater in Texas. Since areas between held points in the adjustment are interpolated, there is no mathematical conversion but the National Geodetic Survey (NGS) software VERTCON will give an approximate transformation. Any printed list of coordinates, any file, or any map showing elevations, must indicate the appropriate vertical datum and the unit of length.

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 digits designating the county number followed by four digits to be used as the discreet control point number (which is not to be duplicated in the county).

A separate data sheet for each newly set bench mark used for control should be prepared showing the quality of the point and the tie points it was surveyed from as well as the usual information associated with control points. The TxDOT Control Point Data Sheet is meant to convey all necessary information for anyone to recover and use this monument at any time without any doubts or further research.

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