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 Classification, 
 Standards of Accuracy, 
 and General Specific- 
 ations of Geodetic 
 Control Surveys 
 
 U.S. Department of National Oceanic and National Ocean 
 
 Commerce Atmospheric Administration Survey 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 LYRASIS Members and Sloan Foundation 
 
 http://archive.org/details/classificationstOOnati 
 
Classification, 
 Standards of Accuracy, 
 and General Specific- 
 ations of Geodetic 
 Control Surveys 
 
 Prepared By: 
 
 Federal Geodetic Control Committee 
 
 U.S. Department of Commerce 
 
 Frederick B. Dent, Secretary 
 
 National Oceanic and Atmospheric Administration 
 Robert M. White, Administrator 
 
 National Ocean Survey 
 Allen L. Powell, Director 
 
 L 
 
 Rockville, Md. 
 
 February 1974 
 a 
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 in 
 
Contents 
 
 Introduction 1 
 
 Horizontal control 2 
 
 Vertical control : 8 
 
 National Networks of Geodetic Control 10 
 
 Tables 
 
 1. Standards for the classification of geodetic control and principal recom- 
 mended uses 3 
 
 2. Classification, standards of accuracy, and general specifications for hori- 
 zontal control 4 
 
 3. Classification, standards of accuracy, and genreal specifications for ver- 
 tical control 9 
 
 4. National Geodetic Networks 12 
 
 hi 
 
Classification, Standards of Accuracy, and 
 General Specifications of Geodetic Control 
 
 Surveys 
 
 Introduction 
 
 The Government of the United States makes na- 
 tionwide surveys, maps, and charts of various kinds 
 which must be referenced to national datums. These 
 are necessary to provide basic information for the 
 conduct of public business at all levels of govern- 
 ment, for planning and carrying out national and 
 local projects, for programs relating to the develop- 
 ment and utilization of natural resources, for Na- 
 tional defense, and for development of the country. 
 Requirements for geodetic control surveys are most 
 critical where intense development is taking place; 
 included are offshore areas, where the surveys are 
 used in the development and exploitation of the 
 marine resources and in the delineation of state and 
 international seaward boundaries. 
 
 State and local governments and industry regularly 
 cooperate in various parts of the total surveying 
 and mapping program. In making surveys and maps 
 of large areas, it is first necessary to establish frame- 
 works of horizontal and vertical control surveys. 
 These provide a common basis for all surveying and 
 mapping operations, and so ensure a coherent 
 product. 
 
 Geodetic surveys of large areas are affected by 
 and must take into account the curvature of the 
 earth, astronomic observations, and gravity deter- 
 minations. 1 Geodetic surveys, executed with high 
 precision, are used to control mapping and charting 
 operations as well as engineering projects. "Geodetic 
 survey" and "control survey" are terms used almost 
 interchangeably. 
 
 Control surveys are of two types: horizontal and 
 vertical. Horizontal control surveys determine lati- 
 tudes and longitudes referenced to a national datum 
 and provide the basis for rectangular coordinate 
 systems. Horizontal geodetic surveys are adjusted 
 to the mathematical figure of the earth applicable 
 
 1 The standards of accuracy for gravity determinations and observa- 
 tions of astronomic latitude and longitude are not contained herein 
 as they depend on the particular application involved; they are de- 
 scribed in separate publications such as Coast and Geodetic Survey 
 Special Publications No. 237, "Manual of Geodetic Astronomy, De- 
 termination of Longitude. Latitude and Azimuth," 1947, GPO, Wash- 
 ington, D.C. 
 
 to the national datum. Vertical control surveys de- 
 termine elevations referred to a national datum that 
 has been referenced to tidal measurements. Vertical 
 geodetic surveys are adjusted with respect to the 
 geoid, an equipotential surface of the earth approxi- 
 mated by mean sea level. These surveys provide 
 permanently marked and properly described stations. 
 
 Horizontal control is established by triangulation, 
 trilateration, and traverse procedures. Triangulation 
 is a system of joined or overlapping triangles in 
 which the length of an occasional side, known as a 
 base line, is measured and the other sides are com- 
 puted from angles measured at the triangle vertices. 
 Trilateration is a method of surveying in which the 
 lengths of the triangle sides are measured. Traverse 
 is a method of surveying in which a sequence of 
 lengths and directions of lines between points on 
 the earth are measured and used in determining 
 positions of the points. 
 
 Vertical control is established by leveling of a 
 high order of accuracy. It provides elevations of 
 marked points along lines that form closed circuits 
 and is accomplished by measuring differences in 
 elevation between consecutive bench marks. 
 
 These classifications and standards have been pre- 
 pared by the Federal Geodetic Control Committee 
 (FGCC) and have been reviewed by the American 
 Society of Civil Engineers, the American Congress 
 on Surveying and Mapping, and the American Geo- 
 physical Union. The opinions of other organizations 
 and individuals were also requested and received. 
 After consideration of all comments, the original 
 draft was revised in this, the present form. 
 
 These new classifications and standards replace 
 those approved March 1, 1957 (and referred to in 
 Exhibit C to the Office of Management and Budget 
 Circular A- 16 dated October 10, 1958). The classic 
 First-, Second-, and Third-Order nomenclatures 
 have been retained. However, classes within the 
 orders have been revised to reflect a requirement for 
 greater flexibility in designing surveys and in recog- 
 nition of rapidly changing requirements for higher 
 accuracy surveys. Improvements in the technology 
 in recent years make these changes desirable and 
 practical. 
 
The specifications in the attached tables show the 
 permissible tolerances on the indicated quantities as 
 a function of order and class; these must be very 
 closely followed to insure that the overall standards 
 are achieved. Detailed specifications have been pre- 
 pared by the Federal Geodetic Control Committee 
 to supplement this publication. Some important al- 
 though seemingly minor factors which do not fit into 
 the tables could cause a survey to fail to attain the 
 required closures even though the specified require- 
 ments were met. It is emphasized that closure is 
 not the sole criterion for classifying a survey into 
 a particular order, as the closure is only part of 
 the requirements and must be considered together 
 with the other specifications in classification. 
 
 These classifications and standards are a directive 
 for surveys which are to be considered a part of 
 the National Geodetic Control Networks. Densifica- 
 tion and extension of control is also provided by 
 analytic photogrammetric triangulation and satellite 
 systems (e.g., Doppler). It is highly desirable that 
 these and all other geodetic or precise engineering 
 surveys be properly referenced to the National Net- 
 works and adhere to these standards whenever pos- 
 sible. 
 
 Horizontal Control 
 
 First-order (Primary Horizontal Control) 
 
 The primary framework for National Horizontal 
 Control Network consists of arcs of triangulation 
 as well as trilateration and traverse, spaced about 
 100 kilometers apart in each direction. To maintain 
 satisfactory mathematical consistency within the 
 contained area networks between these arcs, this 
 primary framework should have an accuracy of at 
 least 1 part in 100,000. The study of gradual and 
 secular ground creep movements in the earth's crust 
 in areas subject to seismic or tectonic activity, the 
 testing of defense and scientific equipment, and use 
 in high-precision engineering projects require control 
 of this same accuracy. These surveys that develop 
 the National Network, prepare for metropolitan ex- 
 pansion, and serve these scientific and engineering 
 purposes are designated First-Order. 
 
 Second-Order, Class I (Secondary Horizontal 
 Control) 
 
 This class includes the area networks between 
 the First-Order arcs and detailed surveys in very 
 high value land areas. Surveys of this class strengthen 
 the National Network and are adjusted as part of 
 the network. Thus, this class also includes the basic 
 framework for further densification. The internal 
 closures of this class of survey should indicate an 
 accuracy of at least 1 part in 50,000. 
 
 Second-Order, Class II (Supplemental Horizontal 
 Control) 
 
 The demands for reliable horizontal control sur- 
 veys in areas which are not in a high state of de- 
 velopment or where no such development is antici- 
 pated in the near future justify the need for this 
 classification. This class is used to establish control 
 along the coastline, inland waterways, and interstate 
 highways and is recommended for controlling exten- 
 sive land subdivision and construction. The control 
 contributes to the National Network and is pub- 
 lished as part of the network. The minimum ac- 
 curacy allowable in Class II of Second-Order is 1 
 part in 20,000. 
 
 Third-Order Class I and Class II (Local 
 Horizontal Control) 
 
 Surveys of this order are used to establish control 
 for local improvements and developments, topo- 
 graphic and hydrographic surveys, or for such other 
 projects for which they provide sufficient accuracy. 
 This order of survey is based on higher order con- 
 trol, carefully connected to the National Network, 
 and also should be permanently marked and ade- 
 quately described. Spires, stacks, standpipes, flag 
 poles, and other identifiable objects located to this 
 accuracy also have significant value for many sur- 
 veying and engineering projects. The work should 
 be performed with sufficient accuracy to satisfy the 
 standards listed in the tables. 
 
 Although the specifications for the triangle closure 
 and side checks are better than the minimum re- 
 quired for the standards, the specifications are log- 
 ical and obtainable with judicious procedures; ad- 
 herence to the standards is recommended. 
 
 Standards for surveys below Third-Order are not 
 included in these specifications. 
 
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Vertical Control 
 
 First-Order 
 
 Leveling of this order is used in developing the 
 basic framework of the national vertical net in the 
 United States (Basic Nets A and B) so that few 
 points in the country will be more than 50 km 
 from an established First-Order bench ^iark. All 
 lines close upon First-Order leveling to form cir- 
 cuits. The lines are divided into sections 1 to 2 
 km in length, and each section is leveled forward 
 and backward. The difference in the two levelings 
 must not exceed 3.0 mm (K) 1/J for Class I (Basic 
 Net A), or 4.0 mm (K) 1 ^ for Class II (Basic Net 
 B), where K is the distance in kilometers. The same 
 criteria are recommended for use in establishing 
 primary networks of leveling in metropolitan areas, 
 except that the lines should be closely spaced. 
 
 Actual gravity values at the bench marks are 
 needed to compute geopotential differences. If the 
 gravity is not already available with the required ac- 
 curacy, it shall be measured at sufficient number of 
 bench marks so that the gravity uncertainty com- 
 puted for any interval will not affect the accuracy 
 of the geopotential difference by more than 0.2 x 
 10- 3 gpu\ 
 
 Second-Order, Class I 
 
 Leveling of this class should be used in developing 
 the secondary net of the national vertical network 
 and in densifying precise control in metropolitan 
 areas. The leveling should connect to leveling of 
 equal or greater accuracy to form closed circuits. 
 All lines should be divided into sections 1 to 2 
 km in length, and each section should be run for- 
 ward and backward, the two runnings of a section 
 not to differ more than 6 mm (K) 1 /-, where K is 
 the length of the section in kilometers. 
 
 Second-Order, Class II 
 
 This class should be used in subdividing loops of 
 First-Order and Second-Order, Class I leveling to 
 establish general area coverage. The leveling should 
 form closed circuits with leveling of equal or greater 
 accuracy, and should rarely extend more than 50 
 km unsupported in this manner. Single-run leveling 
 for short distances is acceptable, but for distances 
 greater than 25 km double-run leveling is recom- 
 mended.. For double-run leveling, the line should be 
 
 1 1 geopotential unit (gpu) = 1 kilogalmeter (10 3 cm 2 sec- 2 ) 
 
 divided into sections of 1 to 3 km, and the forward 
 and backward running of each section should differ 
 by not more than 8 mm (K) 1/J , where K is the 
 distance in kilometers. 
 
 Third-Order 
 
 Third-Order leveling may be used in subdividing 
 loops of First- and Second-Order leveling, where ad- 
 ditional control is required for local development. 
 Third-Order lines may be single-run, but must al- 
 ways be loops or circuits closed upon lines of equal 
 or higher order with a check of 12 mm (K) 1/J , or 
 better, where K is the length of the line in kilometers. 
 It is recommended that single-run lines be limited 
 to 10 km in length, and double-run, to 25 km. Ex- 
 ceptions would be control for topographic mapping 
 at a scale of 1:24,000 or smaller, and leveling in 
 mountainous areas, where accuracy requirements 
 may permit Third-Order lines 50 km long. 
 
 Leveling of Lower Order 
 
 Trigonometric leveling, barometric leveling, and 
 fly leveling may be considered as Fourth-Order, 
 or less; standards for these surveys are not included 
 in these classifications. Elevations are normally pub- 
 lished as part of other data. 
 
 Instruments and Procedures 
 
 For First-Order leveling, an automatic or Lilting 
 level with parallel plate micrometer, or equivalent, 
 should be used. It should have horizontal sensitivity 
 of 0.25 second of arc, or better, and should have 
 high-quality optics that will permit repeat reading 
 of 0.2 mm on a geodetic rod at a distance of 50 m 
 under normal atmospheric conditions. The instru- 
 ment should remain stable in a moderate breeze (up 
 to 20 km/h), and should be temperature compen- 
 sated. The rod should be composed of an invar 
 scale under tension on a wood or metal frame 
 equipped with a bull's-eye bubble. The scale should 
 be accurate overall to 0.1 mm. The rods are used 
 in pairs, each rod alternating as the forward and 
 backward rod, and the same rod is always read 
 first regardless of position. That is, one rod will 
 be used for the back readings on odd-numbered in- 
 strument stations, and for the forward readings on 
 even-numbered stations. The lengths of sights should 
 not exceed the criteria given in table 3, and should 
 be shortened if refraction or scintillation is trouble- 
 some. Balancing of forward and backward sights 
 also shall conform to the limits given in table 3. 
 

 
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For Second-Order leveling, parallel plate or sim- 
 ilar micrometers on the instruments are not required 
 but are desirable. Otherwise, three-wire geodetic 
 spirit or automatic levels with horizontal sensitivity 
 of 0.5 seconds of arc are recommended. Stadia 
 should be read for keeping the sight lengths balanced 
 and for adjustment purposes. The rod scale should 
 be accurate overall to 0.2 mm. For Third-Order 
 leveling, it is necessary to read only the middle wire 
 when using the three-wire geodetic spirit level. 
 
 National Networks of 
 Geodetic Control 
 
 There is a continuing basic program for establish- 
 ing, upgrading, and maintaining of the National Geo- 
 detic Control Networks described in these classifi- 
 cations to provide adequate spacing as well as suffi- 
 cient strength and accuracy to meet the needs and 
 satisfy the requirements of engineers and scientists 
 engaged in the development and conservation of the 
 resources of the United States. The accuracy and 
 spacing of the National Network control must be 
 based on the most stringent requirements to be 
 placed upon it; this accuracy should be the best 
 achievable considering economic advantage and the 
 capabilities of instruments and technology. 
 
 Horizontal Control Network 
 
 National overall accuracy is derived through the 
 establishment of a fundamental backbone of the 
 high-precision traverses, with accuracies of 1 : 1 ,000,- 
 000 integrated with satellite triangulation. Primary 
 horizontal control (First-Order, accuracy of 1:100,- 
 000) establishes the principal network with arcs or 
 traverses at a spacing not in excess of 100 km. Sec- 
 ondary horizontal control (Second-Order, Class I, 
 accuracy of 1:50,000), together with supplemental 
 work (Second-Order, Class II, accuracy of 1:20,- 
 000) breaks down the principal network and 
 strengthens the whole. Local horizontal control 
 (Third-Order, Class I, accuracy of 1:10,000, and 
 Third-Order, Class II, accuracy of 1:5,000) is ref- 
 erenced to the network. To meet these accuracy 
 criteria and for optimum utilization of the network, 
 approximate station spacing, in general, along First- 
 Order arcs is 15 km, with a breakdown to 10 km 
 for Second-Order, Class I and 5 km for Second- 
 Order, Class II. In areas of high land value, the 
 station spacing for First-Order is a maximum of 
 8 km and Second-Order, Class I, 3 km. Second- 
 
 Order, Class II and all Third-Order, Class I and 
 Class II are as required for local usage. 
 
 In addition to the above spacing of stations, con- 
 trol shall be established at all airports, towns of 
 2,000 or more population, colleges, and at a 6- 
 to 8-km spacing along coastlines and interstate high- 
 ways. Although wider spacing may suffice for federal 
 topographic mapping, closer spacing is required for 
 surveys of property, highways, transmission lines, 
 reclamation projects, and numerous other engineer- 
 ing activities. Such stations should be situated so 
 they are readily available to local engineers and 
 surveyors. Frequency, stability, recoverability, and 
 accessibility are factors to be considered when em- 
 placing marks (including underground marks, refer- 
 ence marks, and witness posts when appropriate). 
 
 Nationwide High Precision Traverses 
 
 These traverses provide scale for the worldwide 
 satellite triangulation network and upgrade the scale 
 and orientation of the National Network of Hori- 
 zontal Control. They consist of a series of high-pre- 
 cision length, angle, and astronomic azimuth deter- 
 minations running approximately east-west and 
 north-south through the conterminous states, forming 
 somewhat rectangular loops. Smaller loops and spur 
 traverses are added to connect satellite triangulation 
 stations and areas of special interest. 
 
 Standards of accuracy for high-precision traverses 
 are not given herein. Supporting specifications ap- 
 proved by the Federal Geodetic Control Committee 
 outline procedures indicating the care required to 
 obtain the approximate 1 part in 1,000,000 ac- 
 curacy. 
 
 The high precision traverses provide a geodetic 
 reference framework of continental extent which, 
 when remeasured in the future, will provide data for 
 studies of deformation of the earth's crust, including 
 continental drift and spreading. 
 
 Triangulation, Trilateration, Traverse, and Bases 
 
 Methods of establishment of horizontal control are 
 triangulation, trilateration, and traverses, either sep- 
 arately or in combination. This publication does not 
 differentiate' between the methods used. Appropriate 
 instrumentation and procedures are to be selected to 
 satisfy the standards in the tables and to obtain the 
 precision required. 
 
 The general availability of electronic distance- 
 measuring equipment has practically eliminated the 
 use of taping procedures for the measurement of 
 
 10 
 
base lines or traverse lengths. Accuracies are com- 
 parable or superior to those obtained with invar 
 tapes. 
 
 Vertical Control Network 
 
 In leveling, inasmuch as survey errors propagate 
 at least as the one-half power of the distance sur- 
 veyed, and because of the great continental distances 
 in the United States, the precision of the primary 
 network measurements must be of high order. 
 
 The national vertical control network provides for 
 this by a framework of high-precision basic control 
 supporting a secondary network, which in turn sup- 
 ports area control of a density convenient for users. 
 
 The framework consists of two interrelated sys- 
 tems, designated as Basic Nets A and B, covering 
 the country. Basic Net A is composed of lines of 
 First-Order, Class I leveling, forming more or less 
 rectangular circuits 100 to 300 km across, and is 
 the fundamental reference system. Basic Net B is 
 composed of lines of First-Order, Class II leveling, 
 subdividing the circuits of Net A to provide an 
 overall spacing of 50 to 100 km. The Secondary 
 Network consisting of lines of Second-Order, Class 
 I leveling spaced 25 to 50 km apart densifies the 
 national network. The more closely spaced Second- 
 Order, Class II area control is referenced to the 
 network. 
 
 The surface of our continent is constantly chang- 
 ing because of tectonic and other physical forces. 
 Large areas are undergoing subsidence owing to re- 
 moval of ground water or petroleum resources, re- 
 gions are still emerging from glacier recession, ele- 
 vations are changing because of natural movement 
 of the earth's crust, and seismic activity is continu- 
 ous. Engineers and planners need up-to-date eleva- 
 tions to cope with new water levels and crustal 
 changes. 
 
 One of the most important factors in the devel- 
 
 opment of a control level net is to establish marks 
 that will remain stable. In some areas, the causes 
 of ground movement are so deep-seated that it is 
 difficult to establish a mark that will remain stable 
 indefinitely. The usual practice is to establish, at 
 1- to 3-km intervals along lines of First- or Second- 
 Order leveling, bench mark disks set in concrete 
 posts, stable structures, and outcropping bedrock, 
 or secured to rods and pipes driven to refusal (or 
 a stable stratum). At each intersection of lines of the 
 basic framework, a cluster of three "super* 1 marks is 
 established; this cluster consists of deep isolated-pipe 
 marks or marks in bedrock. Intersections of lines 
 of the Secondary Network are marked by clusters of 
 three driven-rod marks. In both cases, the next 
 adjacent regular bench mark on each level line is 
 the driven-rod or outcropping rock type. This pat- 
 tern permits, to a degree, a check on the stability 
 of marks for future extensions. 
 
 Earth Movement Surveys 
 
 Surveys for the measurement of horizontal and 
 vertical movements of the earth's crust are under- 
 taken in areas of known or suspected subsidence or 
 seismic activity, where the safety of the inhabitants 
 and the economy of the region are involved. These 
 surveys consist of periodically repeated precise 
 measurements to provide information relative to 
 crustal distortions and strain buildup for use in 
 geophysical studies and in engineering design and 
 maintenance. Similar surveys, particularly vertical, 
 are required in areas of subsidence caused by with- 
 drawal of underground resources. These repeat sur- 
 veys are necessary to maintain the quality of the 
 networks and to correlate between changes in local 
 mean sea level and crustal distortion. The economic 
 and engineering impact, and the rate of movement, 
 dictate the period between surveys. 
 
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