Chapter 3: Control Points

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Section 1: Control Points

Overview

Definitively established points of known location are essential for project data to be referenced to ground conditions in the future. This section goes over the requirements for establishing these points to ensure that project data retains its utility throughout the project life cycle and beyond.

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Primary Control Points

Primary control points will be referenced to the National Spatial Reference System (NSRS) through Continuously Operating Reference Stations (CORS) or Federal Base Network (FBN) Stations. Primary control points shall not be more than approximately two (2) miles apart and shall be set in intervisible pairs to provide redundancy. Primary control points shall meet the Level 2 accuracy standards as listed in the TxDOT Survey Levels of Accuracy. Surveyors are responsible for knowing whether their equipment and methods can meet these standards.

Surveyors shall endeavor to ensure that control covers the entirety of the project area in a stable planar configuration as shown in the typical control diagrams. A control layout, template provided in the TxDOT Surveyors’ Toolkit, shall be provided to the TxDOT Survey Coordinator prior to control point monument construction.

Construction of primary control monuments shall be done with the intent that they survive for future projects. Whenever possible, they shall be in TxDOT right of way and situated away from likely construction. Construction methods and materials shall match the diagrams and processes outlined in the TxDOT Monument Specifications listed in the TxDOT Surveyors’ Toolkit for primary control monuments.

All primary control points shall be documented on a form ROW-S-2462 Primary Survey Control (in the TxDOT Surveyors’ Toolkit) and shall be signed and sealed by a Texas Registered Professional Land Surveyor or Professional Engineer. These control data sheets shall be stored and indexed by the TxDOT District Survey Coordinator for future projects and open records requests. Primary control points shall also be included on Plans, Specifications, and Estimates documents and any ROW Mapsheets developed for the project. These control points shall also go into any computer-aided drafting (CAD) graphics files generated for the project.

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Secondary Control Points

Secondary control stations shall meet TxDOT Survey Level 3 Specifications. All secondary control points shall reference and adjust to nearby primary control points as directed by the TxDOT District Survey Coordinator.

These should be intervisible at a maximum of 1,500' distance apart, spaced evenly across the project area as much as possible, as shown in the typical control diagrams. A control layout, template provided in the TxDOT Surveyors’ Toolkit, shall be provided to the TxDOT Survey Coordinator prior to control point monument construction.

Construction of secondary control monuments shall be done with the intent that they survive until the end of the project. Whenever possible, they shall be in TxDOT right of way and situated away from likely construction. Construction methods and materials shall match the diagrams and processes outlined in the TxDOT Monument Specifications listed in the TxDOT Surveyors’ Toolkit for secondary control monuments.

All secondary control points shall be documented on Plans, Specifications, and Estimates documents and any ROW Mapsheets developed for the project.

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Aerial Control Points

Aerial control points shall meet TxDOT Survey Level 3 Specifications. All aerial control points shall reference and adjust to nearby primary control points as directed by the TxDOT District Survey Coordinator.

These shall be placed at the direction of a photogrammetrist to meet the precision requirements listed in the Design Surveys chapter of this manual. Aerial Panel Layout, template listed in the TxDOT Surveyors’ Toolkit, shall be provided to TxDOT Survey Staff prior to aerial data collection.

Construction of aerial control monuments shall be done with the intent that they survive until the end of aerial data collection. If possible, they shall be in TxDOT Right-of-Way and situated away from likely construction. Construction methods and materials shall match the diagrams and processes outlined in the Photogrammetry Guide located in the TxDOT Surveyors’ Toolkit.

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Coordinate Systems and Datum

Federal and state mapping products are referenced to two standard coordinate systems: the North American Datum of 1983 (NAD 83) for horizontal positions and ellipsoid heights, and the North American Vertical Datum of 1988 (NAVD 88) for orthometric heights. Surveys are referenced to these datums through measurements to control points of the National Spatial Reference System (NSRS). All control coordinates will be provided in surface and grid, using surface adjustment factors posted in the TxDOT Surveyors’ Toolkit.

An adjustment was done in Texas using Global Positioning System (GPS), which resulted in the 1993 High Accuracy Reference Network (HARN). The network was extended to nearly all old, conventionally surveyed federal monumentation. Projects should be referenced to the published HARN coordinates of National Geodetic Survey (NGS) monumentation.

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Units

Unless otherwise instructed, latitude and longitude will be presented as degrees, minutes, and seconds. Direction indicators N or W will prefix the value and seconds will be carried out five places right of the decimal where accuracy is to approximately 0.001 feet.

Units of length will be in U.S. Survey Feet in all deliverables. Horizontal coordinates should be carried out to 0.01 feet, unless otherwise instructed.

Conversion from meters to U. S. Survey Feet must be made using the following formula:

Meters * 3937/1200 = U. S. Survey Feet

The factor is 3.280833333333 and working with State Plane Coordinates (SPCs) in the millions, one must carry the factor out to 12 places to the right of the decimal as shown.

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Conversions and Transformations

TxDOT will not accept any datum transformations for control. If a change to the NAD83 project datum is needed for comparison of old surveys, two (2) control points can be resurveyed (GPS or conventional) from references in the new datum. Then, a translation-rotation-scale can be done holding to the two (2) points common to both datums. Or, if original raw GPS data is available, it can be reprocessed holding the new datum coordinates rather than the original datum coordinates.

Both CORPSCON and AASHTOW Survey Data Management Systems (SDMS) are acceptable software programs for the mathematical conversion of:

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Metric to US Survey Foot (or visa-versa)

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SPC zones to adjoining SPC zones

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Universal Transverse Mercator (UTM) to SPCs and

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Latitude/Longitude to SPCs.

In addition to these strictly mathematical conversions, these software programs provide useable combined adjustment factors (CAF) at the specific location of a point if the elevation is included in the input.