Section 3: Local and Network Accuracy
Anchor: #i1000083Standards
The new standards support both local and network accuracies:
- The local 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 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 network 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 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.
Anchor: #i1000126Positional Tolerance and Associated Coordinates
On the other hand, someone constructing a GIS will often need some type of positional tolerance associated with a set of coordinates. Network accuracy measures how well coordinates approach an ideal, error-free datum. The following two tables are reproduced from National Geodetic Survey’s “Guidelines for Geodetic Network Surveys Using GPS,” 5/15/00 - Preliminary DRAFT.
In the following table, the word “Expected” is used because the values given are estimates.
Table 3.1 Expected Survey Accuracies (2σ) for NGS Control
|
Positional Component |
FBN/CBN |
UDN |
Height (2cm) |
Height (5cm) |
|---|---|---|---|---|
|
Horizontal Position |
1 cm |
various |
1 cm |
2 cm |
|
Ellipsoidal Height |
2 cm |
various |
2 cm |
5 cm |
|
Orthometric Height |
3 cm |
various |
2 cm |
5 cm |
The table below provides positional accuracy history.
Table 3.2 History of Positional Accuracy (σ) for NGS Control
|
Network |
Time Span |
Network Accuracy |
Local Accuracy |
|---|---|---|---|
|
NAD27 |
1927-1986 |
10 meters |
First Order (1:100,000) |
|
NAD83 |
1986-1991 |
1 meter |
First Order (1:100,000) |
|
HARN |
1991-1997 |
0.1 meter |
B Order First Order (1:1,000,000) A Order (1:10,000,000) |
|
CORS |
1996-Present |
0.01 meter |
0.01 meter* |
* The best value NGS has; so they are assumed to be zero.
Anchor: #i1000149Federal Geodetic Data Committee (FGDC) Standards
Based on the Geospatial Positioning Accuracy Standards, the FGDC-STD-007-1998, Part 2: Standards for Geodetic Networks prepared by the FGDC, the following accuracy standards supercede and replace the accuracy standards found in FGCC 1984 and FGCC 1988 (see Appendix A, References). The classification standard for geodetic networks is based on accuracy.
The table of accuracy standards for horizontal, ellipsoid height, and orthometric height is as follows:
|
Accuracy Classification |
95 % Confidence |
|---|---|
|
1- Millimeter |
0.001 meters |
|
2-Millimeter |
0.002 meters |
|
5-Millimeter |
0.005 meters |
|
1-Centimeter |
0.010 meters |
|
2-Centimeter |
0.020 meters |
|
5-Centimeter |
0.050 meters |
|
1-Decimeter |
0.100 meters |
|
2-Decimeter |
0.200 meters |
|
5-Decimeter |
0.500 meters |
|
1-Meter |
1.000 meters |
|
2-Meter |
2.000 meters |
|
5-Meter |
5.000 meters |
|
10-Meter |
10.000 meters |
Anchor: #i1000167
TxDOT Standards
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 less accuracy than the A and B order federal monuments, which 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.
|
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 |
Wing Panels for Horizontal Positions*, 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) |
* Vertical positions for wing panels fall in Level 3. However, if panel elevations have been determined by differential leveling, horizontal positions, if needed, can be determined with Level 4 GPS observation.
In the interest of keeping its position information integrated properly with the NSRS, TxDOT will require all new Level 1 (“B” order densification) control, which is established to be referenced to a minimum of four (4) of the closest A or B order stations surrounding the station.
Two of these may be publicly accessible FBN or CBN stations but at least two must be CORS stations. CORS stations, with their downloadable data, are economical to use and are the most accurate stations available. The ties to the CORS, FBN and CBN stations will serve as the method to determine the network accuracy.
|
Level of Survey Accuracy * |
Level 0 |
Level 1 |
Level 2 |
Level 3 |
Level 4 |
|---|---|---|---|---|---|
|
Local Accuracy Class. (m) |
0.02 |
0.02 |
0.05 |
0.10 |
0.10 |
|
Base Error (m) e |
0.008 |
0.008 |
0.010 |
0.010 |
0.010 |
|
Ppm p |
2 |
4 |
10 |
20 |
50 |
|
Minimum baseline length (m) d |
10000 |
3000 |
400 |
150 |
150 |
|
Maximum baseline length (m) d |
20,000 |
10,000 |
10,000 |
5,000 |
3,000 |
|
Maximum allowable error based on minimum baseline length (m) s |
0.022 |
0.014 |
0.011 |
0.010 |
0.013 |
|
Maximum allowable error based on maximum baseline length (m) s |
0.041 |
0.041 |
0.10 |
0.10 |
0.15 |
|
NGS Classification Range(See Table 3.7) |
VI |
VI |
VII |
VII |
VIII |
* This table does not apply to mapping grade levels 5, 6 and 7.
Based on the level of survey accuracy listed in Table 3.4, Table 3.5 may be used as the standard for future classification of surveys performed for TxDOT. The maximum allowable errors listed on the following page are based on:
- All connected and unconnected baselines (vectors) within the minimally constrained and fully constrained network adjustments must comply with the 3-D relative positional error (s) required for the desired level of survey accuracy classification.
Equation for determining maximum relative positional error at the 95% confidence level:
Where,
- s = Maximum allowable relative positional error (m) at the 95% (2σ) confidence level
- e = Base error in meters (m)
- p = Parts per million (ppm)
- d = Distance in meters (m)
New NGS GPS Accuracy Standards:
|
Classification |
Minimum Geometric Accuracy Standard at 2 σ |
|---|---|
|
|
Less than or equal to: |
|
AA |
0.003 m + 1:100,000,000 |
|
A |
0.005 m + 1:10,000,000 |
|
B |
0.008 m + 1:1,000,000 |
|
First |
0.010 m + 1:100,000 |
|
Second, Class I |
0.020 m + 1:50,000 |
|
Second, Class II |
0.030 m + 1:20,000 |
|
Third |
0.050 m + 1:10,000 |
The following table provides the classification range and confidence levels for accuracy standards:
|
Classification Range |
95 % Confidence Level in Meters |
|---|---|
|
Range 0 |
Reserved for CORS |
|
Range I |
< 0.001 |
|
|
|
|
Range II |
0.001 – 0.002 |
|
Range III |
0.002 – 0.005 |
|
Range IV |
0.005 – 0.010 |
|
|
|
|
Range V |
0.010 – 0.020 |
|
Range VI |
0.020 – 0.050 |
|
Range VII |
0.050 – 0.100 |
|
Range VIII |
0.100 - 0.200 |
|
Range IX |
0.200 - 0.500 |
|
Range X |
0.500 - 1.000 |
|
|
|
|
Range XI |
1.000 - 2.000 |
|
Range XII |
2.000 - 5.000 |
|
Range XIII |
5.000 - 10.00 |
|
Range XIV |
> 10.000 ** |
** Ranges larger than XIII will be developed jointly with other subcommittees within FGDC.