A compass survey determines the position of objects using angular measurements from a compass and linear measurements from a chain, tape, or laser range finder. It is one of the oldest field methods in surveying, and it still appears wherever speed matters more than instrument cost. Two instruments dominate the work: the prismatic compass and the surveyor compass. This article compares their parts, least counts, and adjustments, and explains when to use each. The principles of bearings and compass surveying underpin everything described here.
What Is Compass Surveying?
Compass surveying is a branch of surveying in which the directions of survey lines are determined with a magnetic compass, while the lengths of the lines are measured with a tape, chain, or laser range finder. The compass is generally used to run a traverse, a connected series of lines whose bearings and lengths define the shape of a site. At each station the surveyor reads the bearing, chains the distance to the next station, and plots the traverse from the recorded values.
The magnetic compass in surveying depends on the earth’s magnetic field, so the needle always settles toward the magnetic meridian. Every bearing is measured with reference to that direction, which means the surveyor must know the local declination between magnetic north and true north before converting the readings into a map.
How a Compass Traverse Works
The traverse starts at a known point and each station is occupied in turn. At every station the surveyor sights the next point, reads the bearing, and measures the distance. When the traverse closes back on the starting point, the difference between the plotted and actual positions is the closing error, which is distributed across the legs before the final plot is drawn.
When Compass Surveying Is Still Used
Compass traverses are fast and cheap, which makes them useful for preliminary reconnaissance, for filling in details, and for sketching along roads or rivers. They are not used where high accuracy is required, such as precise boundary determination on expensive land, where a theodolite or total station takes over.
Surveyor Compass: Parts, Least Count, and Adjustments
The surveyor compass was originally a colonial instrument designed for use in places such as Ireland and America, where land was plentiful and speed mattered. The first written reference appeared in William Folkingham’s Art of Surveying, published in London in 1610, and the earliest surviving example was made in Dublin in 1667. Most surveyor compasses are made of brass.
In this instrument the graduated ring is attached directly to the box rather than to the needle. An edge bar needle floats freely over the pivot, so the ring is not oriented to the magnetic meridian the way it is in a prismatic compass. The object vane is similar to the prismatic type, and the whole assembly is larger and heavier than a hand held prismatic compass.
Least Count of the Surveyor Compass
The least count is the smallest reading the instrument can resolve. The surveyor compass ring is typically divided to single degrees, and readings are taken to the nearest half degree or better by estimation. The prismatic compass, by comparison, has a least count of 30 minutes, so it can read only observations that are multiples of 30 minutes.
The surveyor compass in detail, including the graduated ring and the edge bar needle, is documented on the Dream Civil reference page, which is a useful cross check when you are comparing the two instruments side by side.
Adjustments of the Surveyor Compass
Like the prismatic instrument, the surveyor compass needs temporary adjustments at each setup and permanent adjustments made in the workshop. The main temporary checks are leveling the box and centering it over the station; the permanent adjustments correct the pivot, the sight vanes, and the leveling bubble.
Prismatic Compass Survey: Parts, Least Count, and Adjustments
The prismatic compass was invented in 1814. It consists of a small circular box about 100 mm in diameter, and it can be used as a hand instrument or mounted on a tripod. It is designed for rough surveys where the accuracy of the survey is not the main consideration but the speed is, and it is useful for filling in details in places where the ground does not allow chaining.
The military uses the prismatic compass for reconnaissance surveys and for sketching along roads or rivers, and a practicing surveyor reaches for it when detail work has to keep pace with the traverse.
Main Parts of a Prismatic Compass
- Box, lifting lever, and magnetic needle.
- Agate cover and glass cover protecting the needle assembly.
- Graduated ring and prism for reading the bearing while sighting.
- Prism cap, sighting slit, and lifting pin.
- Colored glasses for bright sunlight and a focusing screw.
- Object hair vane with horsehair and a reflecting mirror.
- Brake pin and spring brake to damp the needle swing.
Least Count of the Prismatic Compass
The least count of the prismatic compass is 30 minutes, or half a degree. The graduated ring is divided accordingly, and the instrument can read only observations that are multiples of 30 minutes. For finer angular work, a theodolite or total station is required.
Adjustments of the Prismatic Compass
Adjustments fall into two groups. Temporary adjustments are made at each setup and include centering, leveling, and focusing. Permanent adjustments are made in the workshop and correct the needle, the pivot, and the prism.
Temporary Adjustments
Set up the instrument, center it over the station, level the box, and focus the prism so the ring graduation and the target are both sharp. Keep keys, pins, and other metallic objects away from the compass, because local attraction from nearby metal pulls the needle off the true bearing.
Permanent Adjustments
Permanent adjustments fix manufacturing and wear issues: the straightness of the needle, the sharpness of the pivot, the verticality of the sight vanes, and the position of the prism. These are done with the instrument opened on a bench and should only be attempted by someone familiar with the construction.
Prismatic Compass vs Surveyor Compass: Key Differences
The two instruments solve the same problem with different mechanics, and the differences show up in the reading method and the precision you can expect.
| Feature | Prismatic compass | Surveyor compass |
|---|---|---|
| Least count | 30 minutes | 1 degree or less |
| Graduated ring | Attached to the needle | Attached to the box |
| Reading method | Prism viewed while sighting | Direct read over the needle |
| Needle type | Magnetic needle carries the ring | Edge bar needle floats over the pivot |
| Typical use | Reconnaissance, detail filling | Traverse lines, land surveys |
| Character | Speed oriented | More accurate angular readings |
Choosing the Right Compass for the Job
For fast reconnaissance, where the accuracy of the survey is not the main consideration but the speed is, the prismatic compass wins because the reading is taken while the target is still in view. For running a traverse line where the angular readings will be plotted and checked, the surveyor compass is the better instrument.
Whichever instrument you train with, competence in boundary and public land surveys is confirmed through state licensure. The path to a professional surveyor license in Mississippi shows what a surveyor must master before taking responsibility for legal traverses, and the same standards apply in most states.
Running a Compass Survey in the Field
Step by Step Compass Survey Procedure
- Reconnoiter the site and select stations so the sight lines are clear and the legs are roughly equal in length.
- Set up the compass at the first station and level it.
- Sight the next station, read the bearing, and record it in the field book.
- Measure the distance with a chain or tape and record the length.
- Move to the next station and repeat until the traverse closes back on the start.
- Plot the bearings and lengths, then adjust for closing error.
Sources of Error and Local Attraction
The main sources of error are local attraction from nearby metal, incorrect centering, and misreading the least count. Local attraction is avoided by keeping the compass away from keys, pins, fences, and power lines, and by checking the bearing on both the forward and back legs of each line so the difference between them exposes any attraction.
Surveyors who run these traverses for boundary work usually hold a state license, and the route to becoming a licensed land surveyor in Virginia lists the education, experience, and examination requirements in order.
Selecting Survey Stations and Closing the Traverse
Station Selection Criteria
Station selection decides how quickly the survey runs and how accurate the final plot will be.
- Choose stations with clear sight lines to the next point.
- Keep the legs roughly equal in length to balance the error.
- Avoid placing stations near metal objects, fences, or power lines.
- Pick stations that are easy to mark and recover later.
The critical factors in the selection of survey stations go beyond sight lines and include ground stability, the permanence of the mark, and the geometry of the traverse as a whole, because a poorly placed station wastes more time than any instrument error.
Closing the Traverse
When the traverse returns to the starting point, the plotted position will not exactly match the starting position. The difference is the closing error. If it falls within the acceptable limit for the survey class, distribute it across the legs; if it is large, re-measure the suspect lines before plotting.