Surveying, Metal Detecting
and Field Walking
FSARG undertake a number of different types of surveying in connection with the archaeology. Theses are:
When a single large or a series of adjacent excavations take place it is important to establish the height and relative levels of features, contexts and on occasions individual finds. Level surveying is used for this purpose. This is the type of surveying that you’ve probably seen alongside new building or road developments. Level Surveying is also useful in plotting the profile of the ground surface and underlying geology, particularly where extractive workings have interfered with the structure of the land. The type of surveying instrument we use is called a Dumpy Level, this is the simplest and cheapest of precision surveying instruments. This type of surveying is relatively easy to perform and can be quite inexpensive. Complete basic kits can be purchased for around £120 which include the level, tripod and measuring staff.
It is good practice to reference the levels in the excavation to the Ordnance Survey datum (OD), which is the mean sea level at Newlyn in Cornwall. GPS could be used to perform this measurement but anyone who has tried it will know that a standard GPS has poor height accuracy. Establishing the local height of the OD requires a large scale (1:10,000 or 1:2,500) Ordnance Survey (OS) map. These maps show the location of bench marks with their associated height. The photographs above show two different styles of benchmark, they can be challenging to find as they often get covered in vegetation. Unfortunately, with the advent of high technology surveying techniques, the OS no longer maintain the benchmarks and many have gone missing. If a benchmark cannot be found the OS Maps also show spot heights at easily identifiable locations (often the middle of a road junction, take care!).
The key to obtaining accurate results is in the setting up of the equipment. Incorporated into the unit is a circular spirit level vial like the one shown here. It is vital that the equipment is set up completely level. Older dumpy levels will have a fine adjustment mechanism so that it can be adjusted so the bubble sits perfectly in the centre of the circle. Many modern dumpy levels incorporate an automatic self levelling mechanism once the bubble is roughly in the circle. Any error will give a discrepancy in the measurements. This error multiplies the further away the staff or bench mark is.
Once the dumpy is correctly levelled, the measuring staff is positioned at the bench mark or spot height. The cross-hair indicator built into the dumpy level telescope is lined up with the measuring staff and the height marking, that lines up with the cross hair, noted. The dumpy is then rotated and the measuring staff moved to the new location and another measurement taken. The difference in measurements taken at the two points is the difference in the height of the two points. If there is no direct line-of-sight, it will be necessary to make measurements at one or more intermediate points. Moving the tripod to a new position and making pairs of measurements until the feature is reached.
Once all the measurements are taken, a calculation can be made of the actual level. It is quite usual to establish a temporary bench mark at or near the excavation location, so that further measurements can be made in reference to this rather than having to repeat the whole process from the original bench mark. The temporary bench mark can either be transferred to a solid structure such as the top of a low wall, or it can be something such as the top of a stake driven into the ground.
The Measuring Staff
The dumpy level being set-up
Geo-resistivity surveying is an excellent tool that is able to identify possible structures or ditches beneath the ground. Since this is a non-intrusive method of surveying it is possible to survey over quite sensitive areas such as lawns and open grassland where no damage is done.
The way geo-resistivity works is quite simple. An area is gridded out (we usually set out a 20m x 20m grid) using ropes which have 1m markings along their length. The operator walks down each row and stabs the probes (located along the bottom of the frame) into the ground at 1m intervals. The probes pass a very low electrical current through the ground. It the ground is dry then not much current flows, and the resistance to the flow is high. Conversely, if the ground is wet, then the electrical current can pass more easily, and therefore the resistance to the flow is low. A display on the unit shows the resistance reading which is then recorded. The operator then steps forward to the next 1m mark and takes another reading. This process continues until the whole grid has been covered and each point recorded (400 measurements). The grid is then moved to the next area and the whole process repeats until all the survey area is covered.
With each 1m point recorded, the resistance values are entered into a computer software program that produces a pictorial representation of the value. For example, if the resistance value is low, then this may be represented with a dark square, if high, a light square, and anywhere in between by shades of grey (see below). We use a Freeware computer program called Snuffler which is available from here.
When many of these are put together, an image is built up like the one shown here on the right - which is part of Davington cricket pitch. The light areas represent dry and the dark areas wet. The significance of this is that ditches tend to retain water and are therefore wetter, whereas dry areas can indicate the presence of underground structures (such as walls, foundations or demolition materials).
Geo-resistivity surveying in action
Metal Detecting is another excellent tool, used to locate metallic items buried just under the ground surface. Buried metal artefacts can, given good ground conditions, survive for several thousands of years. Metal items can be extremely useful in providing valuable dating evidence. Items such as coins can pin-point dates very accurately – sometimes within a few years span, while other items such as buttons, brooches, furniture fittings and the many other items discarded or lost can, through changes in fashion, design, and material give a very good indication of historic period.
Metal detected finds can provide excellent clues as to the possible activities carried out on a particular site or area, but only forms part of the jigsaw and should be considered as just one aspect of the complete picture.
Metal detectors work in a multitude of ways depending on the type. However, the principle is that a search head is swung across and close to the ground as can be seen in the photo below. The search head emits an electro-magnetic field. When a metal object comes within the vicinity of the field, it has an affect on it. This change is recognised by the machine and is indicated by either visual means or by an audible response.
Metal Detecting in sensitive areas such as this calls for skilled and careful find extraction.
The ‘signal’ can be one of two categories – ferrous or non-ferrous, meaning iron, or everything else. Most modern metal detectors can discriminate between the two. This can be useful for screening out certain signals, but for archaeological work it is usually desirable to recover all types of metal.
Specialist metal detectors can detect down to a substantial depth, but these are not the type normally used for field work. Many metal detectors will detect small coin-size objects at a distance of 12 to 18 inches (30cm - 45cm) in free air, but this is reduced when the item is in the ground due to ‘ground effect’ and other factors. Most metal detected items are recovered within the top 4 inches (10cm) of the surface. This is ideal since we do not want to interfere with any possible archaeology under the ground. In fields where ploughing takes place, the top 12 inches (30cm) will be completely disturbed so digging within this range will not cause any damage to any possible archaeology. Greater care needs to be taken on grass areas or undisturbed land as the archaeology could be much closer to the surface.
Once a signal is detected, it’s then a case of extracting it from the ground. This is carried out with different tools depending on the ground type. For open field areas where the land is usually farmed, a foot trowel is the usual tool as this allows for a quick and easy extraction. When working on grass, then a knife or sharp cutting implement is used. Grass is cut in a special way to allow the item to be removed and the grass replaced with little indication of ground disturbance. The skilled metal detectorist is able to extract metal items buried on lawns leaving absolutely no signs of disturbance.
Once the item has been removed, the finds spot is recorded using a hand-held GPS unit. This gives a good approximation of its location (to within a couple of metres for simple low-cost units). This is then recorded on the bag. GPS is a very good tool when working out in large open areas as the accuracy will be better and by recording the position you will be able to return to the spot very easily. When working in small areas such as gardens, GPS is not really suitable. Gardens are invariably in areas with limited line of sight, reducing the accuracy to a point where the position relative to any archaeology would be meaningless). In these situations, the positions are surveyed / measured in to fixed points of reference.
Metal detectors range in price from just a few tens of pounds up to well over a thousand. Many people who are attracted to metal detecting often buy the low-end machines that have limited functions or performance and as a result have limited success. It’s probably better to buy a second-hand mid-priced machine of an established manufacturer type where success may be higher.
For those who are interested in metal detecting, there are a number of publications available from many newsagents such as 'Treasure hunting' and 'The Searcher'. If you are considering metal detecting, try to join one of the many clubs around the country - not only will you get some great advice and land to detect on, but many can also arrange for public liability insurance – which is essential. If you are unable to join a club, then there are other bodies that you can join such as the NCMD (National Council for Metal Detecting), or the FID (Federation of Independent Detectorists). These will also offer membership with insurance at a very reasonable cost - under £10 per year (as of July 2011).
Whether you choose to join a metal detecting club, an archaeological group such as ours, or work independently, you need to acquaint yourself with the Treasure Act to make sure you operate within the law (Note that this act covers England and Wales). You should also find out who the FLO (Finds Liaison Officer) is for your area and report any finds of interest to them under the voluntary PAS (Portable Antiquity Scheme) , as well as those required by the Treasure Act. FLOs are experts in the identification of metal detected finds and will be only too pleased to assist you. If they can't identify your find, they can often arrange for it be assessed by one of the major Museums such as the British Museum.
Field walking is an exercise used to evaluate an open area of arable land using sampling methods in order to analyse the distribution of datable material. It can help identify 'hot spots' of activity, or to establish particular periods of use. Field walking can also incorporate metal detecting so that items of metal just below the surface can be included with the other dateable material. Although field walking is a structured and disciplined process, it's an activity where practically anybody can take part - adults and children alike. No special skills are needed - just a keen eye.
The field to be walked is divided up into grids (we use 20m x 20m) and marked out using bamboo canes (ideally with a brightly-coloured tops to aid visibility). Each of these grids are assigned a reference number and each person is assigned their own grid. They are given a bag with the grid reference number written on it and stand at the starting point shown below.
When everybody is in place, a whistle is blown and everybody begins. While walking straight ahead, they look at the ground over a 1m wide stretch and pick up any items they see other than natural material. This will include pottery, clay pipe fragments, building material, pieces of glass, plastic, drinks cans, pull-rings, crisp packets etc.
When they reach the end of their 1m section, they turn around and walk back repeating the process on the second 1m wide stretch. They have 10 minutes to complete this task at which point a whistle is blown and everybody stops. If the person has reached the finish point they can carry on going around again covering the same area. The important thing is that everybody has the same amount of time ensuring the results are consistent.
The finds bags are collected and set aside. Each person is then assigned to their next grid and given the appropriate finds bag. This procedure continues until all the field or area being assessed has been covered. If metal detecting has also been conducted, then finds are also placed in bags relating to the grids. In the case of metal detecting, it is more usual to conduct a 100% search rather than 10%.
A total of 40m² of grid is covered out of a total of 400m² representing a 10% sample. There are no hard and fast rules about how much should be sampled, but FSARG find 10% an ideal amount.
When completed, the finds from each grid are processed. After cleaning and identifying, they are weighed and entered into a table which then shows the weight of material for each period (e.g. Roman, Saxon, Medieval, Post Medieval etc.). Each period is then considered across the whole area and for each period, a dot is plotted on the site map for each 10g weight of material found. The diagram below shows an example of this for medieval pottery.
The diagram above shows an example of the distribution of medieval datable material across the surveyed area. Each of the red dots represents 10g in weight of material. It is clear that there is a concentrated area of medieval material in grid references A1, B1, C1, and D1. With results like this it would be possible to concentrate on this particular corner in order to examine in more detail.
Field Walking at Ospringe
Garden surveys (which we refer to as 'foraging') take on a similar approach to field walking. In this case the garden is divided up into grids using an overlay on a street plan. Only areas such as flower beds or vegetable patches (where the soil can be seen) are examined. The grids are obviously smaller than those used for field walking, but the process is similar. Because of the relatively small areas involved, 100% of the area is examined without a time constraint. The results are again tabulated and the different periods of material assessed. This technique is particularly useful when a number of adjacent properties are compared, as was the case during our work in Davington 2010.
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