Techniques: Survey Principles
Introduction
The basic principle of surveying is to be able to work out the position of a point from some other point or points, positioning from the known features to the unknown ones.To start a survey all you do is to measure the distance between two points on the site then draw this to scale on a piece of paper, your site plan. By measuring the distance from these two points to other points on the site the other points can be plotted relative to the two initial points on your plan. You can draw up a network of points joined by distance measurements on your site plan, to scale, as they are on the seabed. That’s it.
Four problems complicate this simple idea:
-
Sites are three-dimensional so we need to deal with differences in height or depth
-
Your measurements will have mistakes in them
-
Your measurements are not perfect
-
Site plans get complicated when there are lots of points and measurements
The simple survey described above works well enough for sites that are flat, as soon as there are any significant differences in depth then it stops working. We need to be able to account for the differences in depth.Expect to make mistakes in your measurements. Previous surveys have shown that between 5% and 15% of measurements made underwater are wrong so we need to add procedures to find these measurements.All measurements are not perfectly accurate, they are all plus or minus a bit where only the size of the 'bit' differs. As we will see later, this measurement error affects the accuracy with which we can position artefacts and ship's structure on our site.
Where we have lots of points to position on a site the number of points and measurements can get too large to deal with easily. To get around this problem we set up a network of points around the site from which we position artefacts and structure. The control point network is a cornerstone of surveying, it looks like yet more work but in fact makes surveying easier.
The Survey Procedure
All survey work follows a simple four-step procedure:
PlanningCollecting MeasurementsProcessing Measurements
Drawing Up
Each step is discussed in detail below:
Planning
The actual amount of time the team will spend on site and underwater is usually very short. It is essential that any work is well planned before it is started so work on site is efficient, safe and problem-free.
The level of detail and accuracy required more or less define the techniques that can be used, these should be specified in the research design. The equipment available and the experience of the team also limit the techniques that can be used but the overriding factor may be the site type.
Collecting MeasurementsBy the time you come to start recording the site the plan should have been explained to all involved. This is the point where teamwork and communication become very important. If the team are well briefed before work and de-briefed afterwards then the team leader can deal with problems as they are found.Any recording forms that have measurements on them should be well looked after. The forms are the primary record of your work and should be kept even after they have been processed. Keeping a notebook with day to day accounts, speculations and ideas about the site is often useful, the notes can be handy later on when the measurements are being processed.
To be efficient the team needs to be well trained and well practised. If any team member needs a refresher then the survey techniques to be used can be practised on dry land beforehand.
Processing Measurements
In the processing stage the measurements that have been collected are used to calculate the positions of the points or detail we are trying to record. Often this process is done while drawing up the site plan if the points are to be plotted by hand.
For some methods distance measurements are corrected for differences in depth. Depth measurements should be corrected for changes in the height of tide during recording. At this stage any mistakes need to be identified and repeat measurements requested.
Drawing Up
With sketches or measurements collected from your site you can start to create the site plan. The plan may be drawn by hand or drawn on a computer. Computer generated plans are more useful as printed copies can easily be made, they can be printed at different scales and the level of detail shown can vary as the work progresses.
If the site has not been recorded before then the sketches or assessment survey results can be turned into a site plan. In doing this you are likely to find that something has been missed and additional work is required.
If a recording survey is being done then the first step is to position the control points around the site on the site plan. Measurements may be needed from the control points to a couple of the main features on the site to align previous site plan with the control points. With the control in place the features can then be added to the plan as they are recorded and processed.
Survey Types
Surveys on archaeological sites can be loosely divided into five types; the end product of each of these types of survey is the same, the site is recorded to known level of detail and known precision.
Assessment Surveys
An assessment survey is one that aims to get a rough idea of the extents and layout of a site, it's like a sketch with measurements. These surveys are used to provide information for rapid assessments of sites, by necessity they must be done quickly. The aim is to get enough information about the site as quickly as is possible. These surveys are sometimes done in advance of a pre-disturbance survey to provide information for planning survey work and control point positions.
Recording Surveys
The most typical type of survey is a recording survey, these include pre-disturbance and excavation surveys. This type of survey requires careful planning, recording and processing so takes time to do correctly. The first step for a recording survey is to set up a set of control points around and inside the site, these points are used as the framework for the survey. Once the control points are in place they can be used to position artefacts or structure or for helping to record details of the seabed.
The foundation of a survey is the network of control points installed on the site, this is the foundation on which the other survey work is built. Permanent, fixed points called control points or control stations are placed around the outside of the site, if the site is large then points may also be installed through the middle as well. The site should be totally covered with simplest possible network of high quality measurements between control points, simple networks are easier to install and position. A single point is not enough to set up position control on a site, a minimum of four points is required. It is essential that the control point network be surveyed accurately, in most cases the control points should be surveyed to greater accuracy than the detail.
The positions of larger features can be recorded by measuring their positions relative to a number of control points. If more than one point on the feature is positioned then the orientation of the feature can also be determined. Over short distances where the highest accuracy is not required offsets and ties can be used, over longer distances or where the high accuracy or position quality is important then DSM or acoustic positioning is used.
Recording Detail and Structure
Where a feature consists of a number of artefacts in a small area then positioning each separately becomes more difficult and can become less accurate. To record a small area to a high resolution then drawing frames or photography are used
Recording the remains of ship's hulls or buildings can be difficult. The remains may be too large to record using drawing frames but too detailed to record by positioning only a few points. Offsets can be useful in recording simple shapes but complex shapes can sometimes only be captured accurately using close-range photogrammetry. Recording the remains of harbours and civil engineering works can be difficult because of the size of the task, techniques used for recording topography are sometimes the most useful.
Monitoring Surveys
Monitoring surveys are a special kind of recording survey where only selected parts of a site are recorded, these surveys are done to monitor changes in the site over time. Not all the site need be re-surveyed each time, only small parts thought most likely to indicate change. A typical example would be to monitor the position, attitude and remains of a stern post on an exposed site or the depth of burial of a site under mud.
Recording Topography
A topographic survey aims to record the shape of the seabed and usually involves recording the seabed type. The same principles are used for recording topography as for other types of survey, what makes this type different is the need to record over a wide area. Recording the shape of the seabed usually involves recording the depth or height of the seabed at known positions. If the measurements are made at regular intervals over the site then a plan can be produced showing the depths as contours.
Survey Techniques
There are a number of techniques that can be used underwater for recording sites. The techniques vary in accuracy, complexity and cost so not all techniques are applicable for all recording tasks :
Sketching |
It is essential to study the site before starting any further survey work and generating a sketch is a good way to do this. |
Radial Surveys |
For a radial survey a diver records the position of features on a site using a distance and bearing back to a single control point. This technique is very simple, quick and can be done by a single diver. The technique is not accurate enough for pre-disturbance or excavation surveys but gives a good enough idea of the site for an assessment. |
Offsets and Ties |
Offsets and ties are used to position features relative to a baseline running through the site. An offset measurement positions a feature using a single measured distance at right angles to the baseline from a known point, a tie uses two or more measurements from known points on the baseline to position the feature. |
3D Trilateration / DSM |
Three-dimensional trilateration or Direct Survey Method (DSM) uses distance and depth measurements to position features on a site. The technique is similar to 2D trilateration with the exception that distances are measured directly to features and the processing deals with any difference in depth. |
Drawing or Planning Frames |
Drawing frames are used to record very small areas of the site in detail. A frame is placed over the area to be recorded and the diver draws visible features on a scale drawing or directly on to the drawing frame. |
Photogrammetry |
Close-range three-dimensional photogrammetry is a very accurate way of recording complex structures. A number of photographs are taken of the structure from different viewpoints and processed in a computer program. The program then constructs a three-dimensional model of the structure to scale. |
If a series of overlapping photographs are taken of a site then they can be joined together to form a single large photograph. The photographs can be of the seabed taken vertically downwards or of the side of a structure. Since the tools to do this have become more widely available this technique is being used more often. Sufficiently good results can be obtained with a digital camera and the graphics processing software available found on most computers. Like close-range photogrammetry and drawing frames, this technique is effective for small areas but must be used in conjunction with a control point network. |
|
Acoustic Positioning Systems (APS) |
Acoustic positioning systems are widely used for survey work in the offshore industry. These systems effectively replace the tape measures and dive computers used for DSM with distances measured with sound pulses. A diver fitted with suitable hardware can be positioned within an array of acoustic beacons, on the surface the archaeologists can then see where the diver is. |