Construction of a simple alidade for estimating NLC elevations

To the right is a simple line drawing which should aid you in the construction of a very simple, but accurate alidade for the measurement of NLC elevations.

Construction is very simple. The materials required for construction are:

- A piece of thin wood with straight edges (check with a set square). Plywood is best due to lightness and strength.

- A bolt or screw with the sharp end filed down to provide a pivot support for the plumb line.

- A home made plumb line (a nut attached to the end of a piece of string works fine).

- A graduated scale. A protractor stuck to the wood works perfectly.

The total cost of materials should be in the rage of a few pounds, and will most likely be considerably less.

Construction notes.

There are no measurements supplied to allow the alidade to be constructed in any size required. The size of the protractor or scale used is a good estimation of the size of the alidade.

Once a piece of wood is selected and accurately cut then the pivot should be positioned at the top centre of the wood (as in the diagram above). The plumb line should then be fixed to the pivot.

The most difficult part of construction is the accurate positioning of the protractor. Inaccurate positioning will cause errors in the elevation measurement. Positioning is achieved by drawing a line at 90° through the pivot to the base of the wood (assuming the wood has straight edges). The 0° axis of the protractor should then be aligned with this line and stuck down.

Using the alidade.

The alidade is even simpler to use than it is to construct. Place the top of the alidade near your eye and sight along the top of the wood to the top or bottom of the NLC display being measured. This is very much like sighting along the tube of a telescope to align it. Once aligned read off the measurement indicated by the plumb line on the protractor scale (this does take a little practice to get right but persevere and it becomes much easier). This measurement is the elevation in degrees. A common error of ±2° is expected on most results.

Improvements to the alidade.

With some adjustments the alidade could be attached to a camera mount to make measurements much easier. The addition of a spirit level to the camera mount can also help to ascertain when the mount and hence the alidade, is level which further reduces measurement errors. This improvement is highly recommended.

I have also constructed an Az-alidade which, when attached to a camera tripod and correctly levelled, allows both the azimuth and a elevation of a display to measured at once to a high degree of accuracy. Contact me for more information if you are interested.