3.3 Survey Regulations: Field Survey Methods and the Standard of Survey
Key Takeaways
- Regulation 3 sets the standard: boundaries shall be defined to an accuracy equivalent to a surround traverse with a closing error of 1:3,000, controlled in direction by connection to Government framework beacons or by astronomical observation for azimuth.
- Regulation 4 requires connection to a Government triangulation or control traverse beacon where any point of the survey is within three miles of one; otherwise a new local origin must be established.
- Regulation 5(A) permits a maximum of thirty stations for closure of bearings, with a maximum misclosure of bearings of 3 seconds times the square root of N.
- Regulation 5(B) requires three separate sun or star observations for azimuth, a maximum range of thirty seconds between them, and a minimum line length of six hundred feet.
- Regulation 5(D) requires field-book entries in ink as measurements are made, with no erasures and no pages torn out, and the field tape error and date of last standardisation recorded.
3.3 Survey Regulations: Field Survey Methods and the Standard of Survey
Source: Survey Regulations made under the Survey Law (regulation-making power, s.18). Regulation 2 defines "survey" for these purposes as any survey made in connection with any plan prepared under the Survey Law for any registrable instrument or judgment or order of court, "beacon" as a permanent mark of any kind made of concrete, iron or stone, including a pillar or boundary post, "town lands" as plots not exceeding ten acres lying within a township, urban district or Government station or within one mile of such a boundary, and "country lands" as all lands which are not town lands.
The standards below are the ones a Folio must meet and a Paper VI answer must cite. They are prescribed in imperial units because that is how they were drafted; quote them as they stand and give the metric equivalent if you wish.
1. The standard of survey (regulation 3)
Subject to such modifications as may be authorised by the Surveyor-General for any particular survey, the survey of the boundaries of any parcel of land shall be effected by methods defining the boundary to an accuracy equivalent to that of a surround traverse with a closing error of 1:3,000, controlled in direction by connection to Government Survey framework beacons or by astronomical observations for azimuth.
Three elements, all examinable:
- The accuracy is 1:3,000. There is no published tiered scheme of urban, sub-urban and rural classes at 1:10,000, 1:7,500 and 1:5,000. If you meet a tighter standard because a client or a project specification demands it, say so as a project requirement, not as the regulation.
- The standard is expressed as an equivalence. "Methods defining the boundary to an accuracy equivalent to that of a surround traverse" - the Regulation does not compel a traverse. GNSS, triangulation, resection or a combination qualify if the resulting definition is of equivalent accuracy.
- Direction must be controlled, either by connection to Government framework beacons or by astronomical azimuth. An internally consistent but unoriented survey does not satisfy regulation 3.
The Surveyor-General's power to authorise modifications for a particular survey recurs throughout Part II, and is the proper route when a project genuinely cannot meet a prescribed method.
2. Connection to control (regulation 4)
Where control exists within three miles (reg. 4(1)). Surveys shall be connected to a Government triangulation or control traverse beacon, or to a previously established local origin, if any point in the survey is within a distance of three miles from such a beacon. Connections may be by triangulation, traverse, theodolite resection, or a suitable combination, to the regulation 3 accuracy.
The Regulation then prescribes how each method must be executed:
- Single triangle from a measured base - the base shall be double chained unless both base terminals are included in the survey; the angle opposite the base shall be not less than fifteen degrees; and where that angle is deduced from observation of the base angles, those angles shall be observed as in regulation 5(A)(1) and repeated on a different zero.
- Connecting traverses - made in accordance with regulation 5 and normally closed circuits. Where closing is impossible, the lengths shall be double chained in opposite directions and a check azimuth observed on the last leg.
- Resections - observed as in regulation 5(A)(1), repeated on a different zero, to at least four favourably situated Government triangulation beacons.
Where no control exists within three miles (reg. 4(2)). The survey shall be connected to a new local origin marked by a permanent beacon constructed as in regulation 10(a), which shall be:
- established in the most prominent accessible position within the area defined by lines one mile outside the boundaries of the parcel; and
- sited between two other permanent beacons.
Except where three of the parcel's own boundary beacons satisfy these conditions and make a connection unnecessary, the connection to the local origin shall incorporate the other two beacons. An azimuth as in regulation 5(B) shall be observed on one of the two lines between the local origin and the other two permanent beacons, and all three beacons shall be described by reference to neighbouring identifiable landmarks.
That last requirement - a written description tying each origin beacon to identifiable landmarks - is what makes the origin recoverable decades later, and it is a standard Folio omission.
3. Standards of measurement (regulation 5)
(A) Theodolite stations
| Item | Requirement |
|---|---|
| Readings at each station | One face right, one face left |
| Maximum number of stations "N" for closure of bearings | 30 |
| Maximum misclosure of bearings | 3" x sqrt(N) |
So for a 25-station chain, the permitted misclosure of bearings is 3 x sqrt(25) = 15 seconds, and a chain may not exceed 30 stations before closure. Face-left and face-right observation is mandatory, which eliminates collimation and trunnion-axis error.
(B) Azimuths
| Item | Requirement |
|---|---|
| Readings at each station | Three separate sun or star observations; unless the latitude is certain to within one minute of arc, observations must be paired east and west of the meridian |
| Maximum range in the three azimuths | 30 seconds |
| Minimum length of line | 600 feet |
The pairing rule is the elegant part. Pairing observations symmetrically about the meridian causes the effect of a latitude error to cancel to first order - which is why the Regulation only relaxes the requirement when latitude is already known to within a minute.
(C) Linear measurements
- Readings to the nearest hundredth of a foot for theodolite traverse.
- Method of taping: along the ground where the nature of the ground and the required accuracy warrant it, otherwise in catenary.
- Corrections to the nearest one hundredth of a foot for standard, slope, and where necessary temperature and sag.
(D) Field books
- All necessary measurements shall be entered in ink as they are made.
- No erasures shall be made or pages torn out.
- Contents shall include: the surveyor's name, title of survey, location, identity of instruments used, error of the field tape and the date of its last standardisation, index and dates of observations, explanatory diagrams accompanying the observations wherever necessary, and measurements to permanent features shown on or alongside the boundaries.
(E) Computations
- Bearings - surveys by theodolite traverse computed on True bearings, or on Colony bearings where connected to points of the main framework of Nigeria, reduced to the nearest thirty seconds.
- Lengths - corrected lengths reduced to the nearest tenth of a foot before computing coordinates.
- Coordinates - computed to the nearest tenth of a foot, by logarithms of not less than five figures or by calculating machine. The Colony coordinates of at least one beacon in the survey shall be computed where Colony coordinates are available for the connecting beacon.
- Areas - the area of land bounded by straight lines shall be computed arithmetically from the coordinates of the corners; the area between an irregular boundary and the adjacent traverse line shall be computed as a series of trapezoids.
- Forms - all routine computations on forms approved by the Surveyor-General, or on good quality foolscap similarly ruled.
The area rule is worth noting: the Regulation requires the coordinate method for straight-sided parcels. Planimetering a plan, or scaling, does not satisfy it.
4. Old datums and disagreements (regulation 6)
Where surveys begin or end on marks of old surveys, the accuracy of the datum shall be assured by angular and linear measurements. On long lines of framework or primary traverse the linear measurements may be dispensed with, but the accuracy of the position of the marks must be checked by angular measurement.
When old marks are found to be out of position, a full report shall be made to the Surveyor-General so that inquiry may be made and steps taken if necessary and possible to correct their positions. A new datum from unmoved marks shall then be selected, or astronomical observations taken to establish a correct bearing.
The instruction is explicit: you do not adopt a suspect datum and note the discrepancy. You select a new datum from marks that have not moved, or you observe an azimuth.
5. Boundary geometry (regulations 7 to 9)
- Regulation 7 - coterminous boundaries. Where a boundary side of a new survey is coterminous with a longer boundary side of a previously surveyed area, the boundary mark or marks of the new survey shall be placed exactly on line between the boundary marks of the old area, so that the old area is not affected by the new survey. This is the regulation that prevents a new survey from silently re-shaping its neighbour.
- Regulation 8 - angular points. Boundaries shall be laid out and surveyed with as few angular points as local conditions and the requirements of the survey necessitate.
- Regulation 9 - irregular boundaries. Where a boundary such as one caused by a stream is irregular, traverse lines shall follow approximately the course of the boundary and offsets shall be taken at suitable points along the traverse to determine the boundary's position.
What accuracy does regulation 3 of the Survey Regulations require for the survey of the boundaries of a parcel of land?
Under regulation 5(A), a theodolite chain of 16 stations is run for closure of bearings. What is the maximum permitted misclosure of bearings?
Regulation 5(B) requires that azimuth observations be paired east and west of the meridian in one circumstance. Which?
A surveyor begins a new survey on marks of an old survey and finds those marks are out of position. What does regulation 6 require?