Measuring how high a hill rises or whether a field is level involves the following equipment:
- A Jacobs Staff — a stick or pole with length markings.
- A set of chaining pins – markers that can be placed in the ground
- A Gunter’s Chain – a steel chain of 100 links exactly 66 feet long
- A plane table – a flat table that can be adjusted so that it is level
- A Sexton or Theodolite (optional and only available to wealthy surveyors) — an optical device for measuring angles to distant objects.
- An alidade — a device that allows one to sight a distant object and determine a straight line
- A spirit level (optional and only available to wealthy surveyors) — a fluid filled vessel with a small bubble situated so that when the flat base of this device is parallel to the flat plain, the bubble is situated between markings indicating level.
- A protractor and plumb bob – a device for measuring angles and their relationship to level.
- String or cording
Determining the elevation between points closer than 1 rod:
This is the method most commonly used by architects and carpenters in the 18th Century but also employed by surveyors. This method only works for short distances.
- Drive stakes into the corners between which you wish to measure elevation differences. Using the plumb bob, ensure these stakes are at a right angle to level (ie 90 degrees on protractor).
- Tie a cord at a known height on each stake (ie use Jacobs Staff to tie at 2 feet). Ensure the cord is taught.
- Aligning the protractor along the cord, measure the angle of the cord using the plumb bob and noting its deviation from 90 degrees (ie an angle of 83 degrees is -7 degrees from level)
- Finally, measure the distance between the two stakes using links of a chain or a marked Jacobs Staff.
- The elevation difference is the Sine of the angle.
Determining the elevation between points 1 rod to 5 chains:
This method is used for both architectural and land surveys but works only when there are no major obstructions and the change in elevation is small. Prior to taking the elevation measurement, you should measure the distances using a chain.
- Establish the plane table above one corner and a Jacob’s Staff at the far corner of the line to which you wish to measure elevation. Use a spirit level of plumb bob and protractor to ensure plane table is level and the Jacob’s Staff is vertical.
- Sight a line with the alidade and determine how high up the Jacob’s Staff a line level with the plane table intersects the staff.
- Measure the height of the plane table and determine the difference in elevation between the two corners.
If the differences in elevation is too large to measure using the method above, and it is impractical to measure points of elevation at interim points, the following method should be employed.
- Establish the plane table above one corner of the line to which you wish to measure elevation. Use a spirit level of plumb bob and protractor to ensure plane table is level. Using the Jacobs Staff, Measure the height of the plane table.
- Place a target mark on the Jacob’s staff at the height of the plane table then place that Jacobs Staff at the far corner of the line for which you wish to measure elevation.
- Release the plane table so that its angle can be adjusted then sight a line with the alidade that intersects the staff at the target. Secure the angle of the table at that angle.
- Measure the angle of the plane table using a plumb bob and protractor.
Determining the elevation between distant points (>5 chains away):
Determining elevation can also be performed optically at any distance for which a line of sight can be established using a theodolite or a sextant. I will provide instructions on this in a future blog.
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