Plane table and alidade survey
Equipment required for a plane table and alidade survey:
• plane table
• spirit level
• 30m tape
• polyester drafting film
• hi-polymer 6H pencil
• masking tape
• scale ruler
• ranging rod for sighting
A plane table (or plain table as it was known prior to 1830) is a simple device used in surveying to provide a solid and level surface on which to make field drawings, charts and maps. The early use of the name plain table reflected its simplicity and plainness rather than its flatness (or plane). The first historical reference to plane table surveys was in the mid-16th century, but it is likely that the methodology was used long before this.
Plane table surveying is a very effective way of getting to grips with all the basic rules of survey to record angles and distances. Modern instruments are designed and constructed on these same fundamental principles. Plane table surveying is best carried out by a minimum of two people, but can be done by a single person depending on such issues as survey area and time constraints.
This method of survey allows a surveyor to plot and create an accurate scale drawing on site. The idea is to scale down your subject matter to a size where it will fit onto your drawing board. A plane table is simply a drawing board fixed to a stand, usually a tripod, levelled and orientated. The table is used, along with a sighting device called an alidade, which is used to observe the points of interest you identify on your site. A simple alidade is basically a ruler with a sight at each end.
Don’t be in too much of a hurry to get started – take time to have a good look at the site or building that you are going to record. On a building, the obvious features that you would want to record are the corners, door and window openings, fireplaces, etc. More subtle features that you would look out for are changes in wall direction, straight joints, blocked doors or windows, changes in the building fabric, etc.
Diagram showing lines of sight from plane table survey stations to points of interest on a standing building. Rather than employing two tables this supposes that Station B was established from Station A initially and then moved – using Station A as its backsight, or point of reference.
This sequence can then, theoretically be repeated around the building.
Selecting a survey station
Once you have satisfied yourself that you understand what it is that you want to record with the plane table, you will need to select a position to set it up; this is called a survey station. Ideally, this location will provide you with a line of sight to as many of the main points of interest as possible. It is particularly important to be able to see easily identifiable features such as corners and other places where a change in alignment might occur. To begin, set up the tripod legs over your selected location.
Select your chosen drawing medium, secure it with masking tape to the drawing board surface and attach it to the tripod. Adjust the tripod so that the board is at a suitable height for the user and is set horizontally using a spirit level. When you have done this you can rotate the board so that its long axis corresponds to that of your building or site as required.
When you are satisfied that you are set up correctly, mark the direction of north onto your plan. If feasible, mark the point on the ground directly under the centre of the table – a plumb-bob will probably be needed for this. If you know that you will need to set up other stations within sight of the first station in order to complete your survey, this is an essential step.
For all plans it is important to know whether the area you are going to survey will fit onto the drawing board at your chosen scale. Check this beforehand by measuring the maximum extents of the survey area. If all the steps have been done to your satisfaction you are now ready to begin surveying!
Firstly, in the method described below, a pin is positioned in the (centre of) the board representing the point on the ground – your survey station. The pin provides a fixed point from which observations and measurements can be taken. Once the survey has begun, the plane table should not be moved; you should be careful when moving around the table not to bump into it or lean on it too heavily.
Place the alidade securely against the pin and rotate it accordingly to observe each of the selected points of interest to be recorded through the sight vane of the alidade. Mark each line of sight around the periphery of the drawing sheet (these are known as rays) with a pencil and record the measurements between the pin and the points taken (measuring tape, red laser distance measurer, etc). It is good practice to write the measurement next to the drawn ray for future reference. Subsequently, each point can plotted along the ray at the selected scale, using a scale ruler.
Take observations and measurements to as many points as required, allowing the shape of your survey to be recorded. These plotted points create a very accurate framework around which the rest of your plan can be constructed. This method is most useful for plans of single buildings or small sites within a radius of 30m from the plane table and where lines of sight are not obscured or angles likely to be too acute.
Larger areas and complex sites
In some circumstances the scale and complexity of a site might require multiple stations to be set up. The second station should be sighted and plotted accurately onto the plan (as a point) and clearly marked on the ground. Ideally, it should be located within easy reach of (less than 30m away from) the first station.