Chapter 3: Preliminary Surveying
Anchor: #i1027023Section 1: Horizontal Control Surveys
Anchor: #i1027031Overview
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.
Anchor: #i1027061Justification
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.
Anchor: #i1027073Resources
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
The TxDOT GPS User’s Manual applies specifically to the use of GPS for performing control surveys.
Anchor: #i1027125Field 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. See the subsection entitled “Fieldwork” under Section 6, GPS State Surveying of this chapter for more information on the use of GPS for horizontal control networks. Also, see chapters 5 and 6 of the TxDOT GPS User’s Manual, which sets out the specifications and procedures for use of GPS for horizontal control.
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.
Anchor: #i1027155Rights 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.
Anchor: #i1027165Monuments
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.
Anchor: #i1027181Azimuth 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.
Anchor: #i1027194Tolerances
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. TxDOT uses Levels of GPS Surveys to differentiate the quality of surveys – see the descriptions of each level in The TxDOT GPS User’s Manual, Chapter 6, Section 6 “Field Survey Operations and Procedures.”
The following table describes the Texas Society of Professional Surveyors (TSPS) Manual of Practice, April 2002 tolerances for conventional traverse:
|
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.
The table below provides survey accuracy information:
|
Level of Survey Accuracy |
Level 0 |
Level 1 |
Level 2 |
Level 3 |
|---|---|---|---|---|
|
Local Accuracy Classification (m) |
0.02 |
0.02 |
0.05 |
0.10 |
|
Base Error (m) |
0.008 |
0.008 |
0.010 |
0.010 |
|
Ppm |
2 |
4 |
10 |
20 |
|
Minimum baseline length (m) |
10000 |
3000 |
400 |
150 |
|
Maximum baseline length (m)* |
200000 |
100000 |
50000 |
12000 |
|
Maximum allowable error based on minimum baseline length (m) |
0.022 |
0.014 |
0.011 |
0.010 |
|
Maximum allowable error based on maximum baseline length (m) |
0.041 |
0.041 |
0.100 |
0.100 |
NOTE: * The maximum baseline length may be longer for Continuously Operating Reference Station, Federal Base Network & Cooperative Base Network (A & B order) ties and these requirements do not apply to network accuracy (positioning on the datum).
Specific information is provided below for GPS network design:
NOTE: * To qualify for a new occupation, the observer must remove the GPS receiver at the station and a completely new setup over that station must take place.
|
Level of Accuracy |
Level 0 |
Level 1 |
Level 2 |
Level 3 |
|---|---|---|---|---|
|
Minimum Number of Closest Direct CORS Ties |
2 |
2 |
1 |
0 |
|
Minimum Number of Total FBN/CBN/CORS Station Ties |
4 |
4 |
2 |
0 |
|
Minimum Number of Horizontal Station Ties |
4 |
4 (Level 0 Ties) |
3 (Level 1 or 0) |
2 (Level 0, 1, or 2) |
|
Minimum Number of Vertical Station Ties (2nd order or better) |
6 |
5 |
4 |
2 |
|
Minimum Number of Occupations per Station |
2 |
2 |
2 |
2 |
|
Minimum Number of Repeat BL’s (% of all BL’s) |
50% |
40% |
30% |
20% |
|
Tim Offset Between Observations (Occupations*) |
± 4 hrs |
± 3 hrs |
± 2 hrs |
± 1 hr |
|
Minimum Satellite Elevation Mask |
15 Degrees |
15 Degrees |
13 Degrees |
13 Degrees |
|
Minimum Number Quadrants for Horizontal Station Ties |
4 |
4 |
3 |
2 |
|
Minimum Number Quadrants for Vertical Station Ties |
4 |
4 |
4 |
2 |
|
Type of Ephemeris Required |
Precise |
Precise |
Rapid or Precise |
Broadcast or Better |
* To qualify for a new occupation, the observer must remove the GPS receiver at the station and a completely new setup over that station must take place.
Anchor: #i1027247The 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.