3.4 Vertical Control, Topographic & Planimetric Mapping (21 NCAC 56 .1605 & .1606)
Key Takeaways
- 21 NCAC 56 .1605 establishes three vertical control classes measured in feet against the square root of the number of miles run from the reference datum: Class A 0.10, Class B 0.20, and Class C 0.30.
- Global positioning surveys shall only be used to obtain Class C vertical surveys, and GNSS vertical error shall not exceed five centimeters relative to the referenced benchmarks at the 95 percent confidence level (2 sigma) under Federal Geographic Data Committee standards.
- 21 NCAC 56 .1606 adopts Part 3 of the FGDC Geospatial Positioning Accuracy Standard for production and Part 1 for reporting, and sets five horizontal classes: AA 0.10 ft, A 1.64 ft, B 3.28 ft, C 6.56 ft, and D 16.40 ft relative accuracy.
- In the absence of a specified standard, a topographic or planimetric survey defaults to the requirements for a Class B survey.
- Ground control for topographic and planimetric projects shall be in North Carolina State Plane Coordinate System grid coordinates and distances when the project is tied to grid, and a minimum of one permanent project vertical control point shall be shown.
3.4 Vertical Control, Topographic & Planimetric Mapping (21 NCAC 56 .1605 & .1606)
These two rules cover everything the Standards of Practice say about elevation and about mapping the ground surface. Both were amended in the 2000s and 2010s to point at federal accuracy frameworks rather than legacy map-accuracy language, and both carry their own certificates.
[!CAUTION] North Carolina's vertical classes are A, B, and C, and the constants are 0.10, 0.20, and 0.30 feet times the square root of the number of miles run. Prep material that presents "Class I / II / III" at 0.020 / 0.035 / 0.050 feet is quoting the federal geodetic leveling classification, which is roughly an order of magnitude tighter and is not the North Carolina rule. Using the federal numbers on the exam will produce wrong answers in both directions — you will fail loops the rule accepts and accept work the rule rejects.
1. Vertical Control Surveys (21 NCAC 56 .1605)
(a) General
Vertical control surveys are defined as "measurements taken by surveying methods (differential leveling, trigonometric leveling, and global positioning surveys) to determine elevation with respect to vertical datum, usually National Geodetic Vertical Datum of 1929 (NGVD29) or North American Vertical Datum of 1988 (NAVD88)."
Then the sentence candidates miss: "Global Position Surveys shall only be used to obtain Class C surveys." GNSS-derived elevations cannot be reported as Class A or Class B vertical control in North Carolina, no matter how good the session.
The three classes
| Class | Maximum vertical error (feet) | Formula |
|---|---|---|
| A | 0.10 times the square root of the number of miles run from the reference datum | C = 0.10 √M |
| B | 0.20 times the square root of the number of miles run from the reference datum | C = 0.20 √M |
| C | 0.30 times the square root of the number of miles run from the reference datum | C = 0.30 √M |
Class C carries an additional GNSS clause: "The vertical error in global position surveys shall not exceed five centimeters relative to the referenced benchmark(s) at the 95 percent confidence level (2 sigma) accuracy as defined in Federal Geographic Data Committee Standards."
+-----------------------------------------------------------------------------+
| WORKED EXAMPLE - CLASS B DIFFERENTIAL LEVEL LOOP |
| |
| A crew runs a closed differential level loop for a municipal storm |
| drainage project to the Class B standard. Total route distance = 9.0 mi. |
| Departing elevation on NCGS BM "OAK" = 342.500 ft |
| Closing elevation back on BM "OAK" = 342.562 ft |
| |
| Step 1 Observed misclosure |
| e = 342.562 - 342.500 = 0.062 ft |
| |
| Step 2 Allowable misclosure, Class B |
| C = 0.20 * sqrt(9.0) = 0.20 * 3.0 = 0.600 ft |
| |
| Step 3 Compare |
| 0.062 ft << 0.600 ft -> PASSES Class B comfortably |
| |
| Step 4 Would it pass Class A? |
| C = 0.10 * sqrt(9.0) = 0.300 ft; 0.062 < 0.300 -> YES |
| |
| Distribute the 0.062 ft across turning points in proportion to distance. |
+-----------------------------------------------------------------------------+
The arithmetic is deliberately forgiving compared with federal geodetic leveling, because .1605 is a land surveying standard aimed at site work, not at extending the National Spatial Reference System. When a project genuinely requires federal-order leveling, that is a contract specification the client imposes on top of the rule — the rule sets the floor.
(b) The certificate
"I, ______________________, certify that this vertical control survey was
completed to the Class ___ standard [21 NCAC 56 .1605(a)] under my direct and
responsible charge from an actual survey made under my supervision."
2. Topographic and Planimetric Mapping (21 NCAC 56 .1606)
The full rule title is "Specifications for Topographic and Planimetric Mapping, Including Ground, Airborne, and Spaceborne Surveys."
(a) Definitions
- Topographic surveys — surveys whose major purpose is determining the configuration (relief) of the earth and the location of natural or artificial objects on it.
- Planimetric mapping — a map presenting horizontal positions only, distinguished from a topographic map by the omission of relief in measurable form.
- Airborne and spaceborne surveys — photogrammetry, LIDAR, IFSAR, or similar technologies for obtaining reliable information about physical objects and the environment, including the terrain surface, by recording, measuring, and interpreting images and patterns of electromagnetic radiant energy.
(b) Production standard
Production procedures shall be in accordance with Part 3 of the Federal Geographic Data Committee (FGDC) Geospatial Positioning Accuracy Standard, incorporated by reference including subsequent amendments and editions, available at no cost from www.fgdc.gov. Reporting accuracy shall be in accordance with Part 1 of the FGDC standards.
[!IMPORTANT] The rule does not contain the old National Map Accuracy Standard language about "90 percent of tested elevations within one-half of the contour interval." That standard belongs to a different framework. North Carolina points at FGDC Parts 1 and 3 and at the class tables below.
(c) The accuracy classes
Maps, orthophotos, and related electronic data — unless marked "Preliminary Map" — shall meet one of the classes below as contractually specified to FGDC standards, USDA NAIP standards, or State adopted base mapping standards. "In the absence of a specified standard, the surveyor shall conform the survey to the requirements for a Class B survey."
| Horizontal class | Relative accuracy |
|---|---|
| AA | equal to or no less than 0.033 meter (0.10 feet) |
| A | equal to or less than 0.5 meter (1.64 feet) |
| B | equal to or less than 1.0 meter (3.28 feet) |
| C | equal to or less than 2 meters (6.56 feet) |
| D | equal to or less than 5 meters (16.40 feet) |
| Vertical class | Applies to | Maximum vertical error |
|---|---|---|
| A | Urban and suburban vertical control surveys — lands within or adjoining a town or city | 0.10 √miles (feet) |
| B | Other vertical control surveys — all lands not covered by Class A | 0.20 √miles (feet) |
| C | Trigonometric vertical control surveys — used for vertical control for aerial and topographic mapping | 0.30 √miles (feet) |
(e) Ground control
"Ground control for topographic and planimetric mapping projects shall be in North Carolina State Plane Coordinate System grid coordinates and distances when the project is tied to grid. A minimum of one permanent project vertical control point shall be shown."
(f) The ten required content items
The project map or report shall contain: (1) date of original data acquisition; (2) altitude of sensor and sensor focal length, as applicable; (3) date of document or data set compilation; (4) for hard copy, a north arrow, map legend, final document scale including bargraph, and contour interval as applicable; (5) coordinate system for horizontal and vertical, denoting SI or English units; (6) a list or note of the control points used, showing for each its physical attributes (e.g., iron rod, railroad spike), latitude and longitude or X and Y grid coordinates, and elevation as applicable; (7) source and accuracy of any other included data; (8) for topographic maps or data sets, contours in areas obscured by man-made or natural features uniquely identified or enclosed by a polygon, noted "No reliance is to be placed on the accuracy of these contours"; (9) a vicinity map on the first sheet or in the accompanying report; and (10) the name of the client for whom the project was conducted.
(d), (g), (h), (i) — the loose ends that get tested
- (d) When the product is a digital data set, or a map or document of more than one sheet that otherwise cannot be certified, a project report shall be certified — marked "Preliminary" if applicable.
- (g) "Nothing in this Rule shall be construed to negate or replace the relative accuracy standards found in Rules .1601 through .1608." A topographic map that also depicts boundary lines is still bound by .1602, .1603, and .1604.
- (h) The certificate recites responsible charge; whether the survey was ground, airborne, or spaceborne; the percent confidence level meeting FGDC standards; the topographic/planimetric class and, where applicable, the vertical class; the date the original data was obtained; the completion date; the note that contours shown as broken lines may not meet the stated standard; and the horizontal datum and realization plus the vertical datum (NGVD 29, NAVD 88, or other).
- (i) Electronically transmitted documents must have the computer-generated seal removed and carry the "originally issued and sealed by ... This electronic media shall not be considered a certified document. See the project report for certificate and seal" language.
3. The Compiled-Boundary Disclaimer
Site-design topographic maps routinely show property lines that were never surveyed. The rule that governs that is 21 NCAC 56 .1604(b), not .1606: "Any lines that are not actually surveyed shall be indicated on the map and a statement included revealing the source of information from which the line is derived."
G.S. 47-30(d) imposes the same duty on recordable plats — "any lines on the plat that were not actually surveyed must be clearly indicated and a statement included revealing the source of information." Whether the drawing is a design topo or a recorded plat, the obligation is identical: mark the line and name its source. Omitting it invites a claim of misleading practice under 21 NCAC 56 .0701(e), which requires that professional reports and statements be objective, truthful, and include all relevant and pertinent information.
A differential leveling loop is run over a total route distance of 9.0 miles for a project requiring Class B vertical control under 21 NCAC 56 .1605. What is the maximum allowable vertical misclosure?
A surveyor establishes elevations across a site entirely with GNSS observations and wants to report them as Class A vertical control under 21 NCAC 56 .1605. Is that permitted?
A client orders a topographic map but specifies no accuracy standard in the contract. Under 21 NCAC 56 .1606(c), what class must the surveyor deliver?
A site-design topographic map depicts property lines compiled from deeds and tax maps rather than from an actual boundary survey. Which requirement governs how those lines must be shown?