11 Survey Methods in Archaeology

K. P. Rao

 

1. Introduction :

 

Archaeology has close relationship with spatial and temporal dimensions. Spatial dimension require that we record the distribution and position of the sites accurately. For accurate recording of the spatial dimensions, we have to use various survey methods. The output, that comes out of such surveys is usually in the form of maps. Maps are physically, two dimensional material, i.e., they have length and breadth, represented as ‘x’ and ‘y’ axis, but not depth or elevation. Hence, we have to use some method in representing the third dimension of elevation. This third dimension, which is known as the ‘z’ axis is usually represented in the form of contours. Contours can be defined as lines joining places having similar characteristics. For example changes in the humidity, temperature, population density, rainfall, land elevation, etc. can be represented using contour lines. Whenever an archaeological site is selected for elaborate study, one of the first studyis thepreparation of contour drawing. Contour drawings are prepared with an aim to understand properly the topography of the site and also to have a proper record of the surface features of the site, before major work like extensive excavations are undertaken. One of the aims of such study is to record the surface features, before they are removed in the process of excavation. Further, the contours also provide us an idea of the lay out of the habitation, if it had some buried features like the stupas, moat, fortification, streets, platforms, etc. Once, we have the elevation data of a site, we can use the data to generate Digital Elevation Model or Terrain Model of the site to examine the elevation features of the site from different angles.

 

2. Elevation Survey and Contour Drawing

 

Definition of  Contour :

 

A contour can be defined as a line joining places of similar characteristics. It could be elevation, rainfall, temperature, humidity, etc.

 

Definition of Digital Elevation Model (DEM) :

 

Digital Elevation Model can be defined as a Three Dimensional representation of a Terrain Surface. A DEM can display x, y and z axis, and the modelling allows us to view the selected area from any angle.

 

3. Basics of Levelling Survey

 

Whenever, we conduct elevation survey, we need a reference point. The readings taken at different points are relative to this reference point. The points known as ‘Benchmark’ serve as the reference points. Usually, the benchmarks indicate the Mean Sea Level (MSL) at that location. Mean sea level is the average of high and low level of the tides observed hourly over an extended period of time. The Benchmarks are usually marked on important buildings, bridges, land marks, etc. Most of the topo maps also indicate Benchmarks at selected locations.

 

3.1 Backsight

 

Backsight or ‘Back sight reading’ is the first point taken after setting up the levelling instrument. A Backsight reading is taken, usually, at a Known Benchmark.

 

3.2 Foresight

 

Foresight or ‘Fore sight reading’ is the last staff reading taken before changing the position of the levelling instrument.

 

3.3 Fixing the Benchmark

 

For conducting any elevation survey, it is essential to have a reference point, which is called a ‘Benchmark’. As stated above, usually, the Mean Sea Level serves as the benchmark. One of the problem encountered while studying an archaeological site is that, the Mean Sea Level (MSL) of a particular location may not be known. If the MSL of the location is to be fixed, then we have to project the reading from a known benchmark.

 

3.4 Projecting a Benchmark

 

To fix the benchmark at a selected place, first we have to start the level measurements from a known benchmark. The levelling instrument is fixed somewhere between the known benchmark and the point selected for fixing the new benchmark. Then Backsight reading is first taken at the known benchmark (point A) and foresight reading is taken at the new benchmark (point B). Deducting the reading of A –B gives the difference in the elevation. For examples point A reading is 2 M and point B reading is 5 M. Then 2 – 5= -3. Suppose the MSL at point A is 230 M, then 230+ -3= 227 M is the elevation at point B. In case it is not possible to project the MSL at the new benchmark in one single operation, we have to conduct the survey in multiple steps. In such situation, the foresight point of the earlier survey becomes the Backsight point for the next stretch.

 

3.5 Elevation Survey

 

The aim of an elevation survey is to record the undulations on the surface of the site. Before undertaking any large scale operation like excavation, the site is first surveyed to properly record its elevation profile. Taking the elevation readings at different levels help us in preparing the contour map or in preparing the Digital Terrain Model. A Digital Terrain Model has many advantages over a contour map, as it can be animated on a computer and we can observe the elevation of the site from any angle of view. This is achieved by generating TIN (Triangulated Irregular Network) mesh of the area. A TIN is made up of irregularly distributed nodes and lines forming triangles based on the x, y, and z coordinate data.

 

3.6 Elevation Survey Using Dumpy Level

 

One of the most common instrument used for elevation survey is the Dumpy Level, also known as Levelling Instrument. A Levelling instrument having automatic line of sight correction facility is known as Auto Level. Auto Level automatically corrects the aberrations, if any, in the horizontal plane. For conducting survey we need the following equipment.

 

3.7 Dumpy Level

 

Dumpy Level or Levelling Instrument is basically an optical telescope with a ‘reticle’ or ‘cross hair’ on the object lens side. We can use the ‘ + ‘ mark in the field of view to take accurate readings. This telescope is mounted on a ‘tribrach’ platform for operating the instrument in a horizontal plane in any horizontal angle.The main parts of a Dumpy level are illustrated below:

 

 

3.8 Setting up the Instrument

 

Usually, the levelling instrument is setup midway between the points which are going to be measured. For setting up the instrument, it is advisable to select stable and compact ground for setting up the instrument. If the soil is too loose, there is the danger of the instrument legs sinking in, which results in erroneous readings. Similarly, if the area is full of small rubble, such material also can result in instability of the instrument. After selecting the place, first we have to extend the tripod stand legs to a height of the surveyor who is going to use the instrument. After tightening the screws on the legs, we have to spread the legs in such a manner that, they provide stable platform for the Dumpy Level. Each tripod leg has to be pressed down firmly into the ground with a foot on the tripod shoe at the bottom of the tripod leg. We have to adjust the height of the tripod legs in such a manner that the tribrach platform maintains horizontal plane. In uneven ground, we may sometimes have to extend or shorten one or two legs. Once, the tripod is fixed firmly, the Dumpy Level instrument has to be mounted on the platform with the help of the screw at the centre. We have to perform the important task of ‘Levelling’ the instrument, so that it provides a perfect horizontal line of sight in any horizontal angle. For this first we have to align the telescope parallel to any two of the foot screws of the tribrach. With the help of the two foot screws parallel to the telescope, we have to adjust the bubble in the bubble level to be on the side of these foot screws. Once we achieve this, then we have to turn the telescope by 90 degrees horizontally, and then adjust the third foot screw to centre the bubble in the circle of the bubble level. Once this is done, the horizontal level of the instrument is adjusted perfectly, and the instrument can provide a perfect line of sight on the horizontal plane.

 

3.9 Level Staff

 

Level Staff is also known as levelling rod. Most of the levelling staff are made either in wood or aluminium. There are various types of levelling staff with different marking methods. It is more convenient to use a rod with metric measurements. The staff is graduated with centimetre markings. Numbers written in Red colour indicate the Meters and the numbers written in Black colour indicate the Centimetres. The Black numbers have to be multiplied with 10 to get the correct measurement. If 2 is sighted (in black), it means 20 Centimetres. In between these 10 cm markings are the centimetre graduations, which are usually not numbered. We have to count these lines starting from the 10 cm markings. The levelling staff has to be held vertically by a field assistant just above the grid nodes/markers. Some levelling staff have bubble levels to aid in maintaining verticality of the staff which is held above the node markers.

 

3.10 Field Marking

 

Before starting the field marking, we have to clear the land of the seasonal growth of plants and bushes. We have to see that an unhindered line of sight is available from the levelling instrument to the staff held above the different nodes. The site we plan to survey has to be, first, marked for undertaking elevation survey. Before undertaking this, we have to decide the resolution at which we want to take the readings. The decision is usually based on how big area we want to survey and second, the precision at which we need the readings. If the area is bigger than 100 x 100 meter area, usually, one meter grid survey would be sufficient. For survey of very large areas 2-5 meter resolution would be sufficient. If we decide to do one meter resolution survey, then, we have to fix markings at one meter gap in grid pattern following cardinal directions. For fixing the grid, first we have to determine the North – South orientation with the help of the Magnetic compass. The markers have to be fixed following the cardinal directions. We have to attach a smalllabel with the grid number to each of the marker.

 

3.11 Grid Numbering

 

We have to mark the grid in X and Y axis. ‘X’ axis runs East – West, while the ‘Y’ axis runs North – South. Usually the numbering starts from the south-western corner.

 

3.12 Field Readings

 

After marking the field, the dumpy level has to be set up at the highest point, either in the field or outside the field, if the area to be surveyed is small. This selected point is the temporary benchmark for doing the survey. Benchmark indicates the elevation of a place with reference to the Mean Sea Level (MSL) at that place. Permanent Benchmarks are usually found marked on the important permanent features like bridges, railway stations and other buildings. The Benchmarks are usually indicated on the Toposheets. We have to determine the elevation of the selected point (point exactly below the instrument), either based on the markings on aToposheet or by using a sensitive GPS (Global Positioning System) instrument. Other procedure is to determine the elevation based on the Benchmark available at the nearest point. But, this later process, though very accurate, is very elaborate, as we have to project and calculate the elevation all the way from the Benchmark. Once, the elevation of the temporary benchmark is known, we have to add the height of the instrument to this elevation to derive the elevation of the line of sight (Eg. Elevation at the point = 33.50 M Height of the instrument = 1.50 M 33.50 + 1.50 = 35 M). We have to note down the elevation of the line of sight in the Field note book.

 

Once, the grid marking and setting up of the instrument is over, then we can start taking readings from the grid points. We can take the reading, once the staff is positioned above a grid node. We may need to adjust the focus of the telescope, initially while taking the first reading. Afterwards, only occasional minor adjustment is sufficient. For pointing the telescope exactly towards the levelling staff, we can take help of the ‘Gun sight’ available above the telescope tube. For synchronizing the vertical reticle line we have to use the horizontal adjusting screw. We have to record the measurement that is corresponding to the centre of the reticle or the cross hair-line. All the measurements have to be noted in the field note book on the field sheet.

 

3.13 Record Sheet

 

A record sheet has to be prepared with five vertical columns divided by horizontal lines. The number of lines depends on the size of the area that is going to be surveyed. A computer printout can be taken in the manner given below for surveying an area of 5 x 5 meters (assuming that the elevation of the line of sight is 35 meters) :

 

We have to enter the Dumpy Level readings in column ‘C’. The actual elevation of the points can be obtained by subtracting this reading from the elevation of the Line of Sight (in column D), which is entered in column E.

 

3.14 Preparing Contour and DEM

 

For preparing contour drawing of the area, we can take advantage of the computer processing. To process the data in computer programmes, first we have to prepare the data suitably. We have to enter the data in the columns A, B and E in a spread sheet like Excel programme (better to save the file in Excel 97-2003 workbook). The file can be opened in a contouring and 3D surface mapping programme like the Golden Software’s Surfer. Once the Programme is opened, we have to click Grid in the Menubar, and in the drop-down menu, we have to select Data. In the popup window, we have to select the Excel file and click open. A pop-up window, Grid Data will come up. Click ‘OK’ option. Using the entries in the data, the programme will prepare a Grid file. The programme says a Grid file has been prepared using the data in the saved file. It automatically saves it with the same name with ‘grd’ Extension. Close the two pop-up windows. This Grid file can be used to view contours, wireframe, surface or many other forms of surface views by clicking Map option in the Menubar. Even if we do the grid survey in higher or lower resolution, still there are options to adjust the interval of the contours in the programme. We can also have multi-colour or single graduated colour rendering for various levels of contours. The Wireframe is a 3D rendering facility, which allows us to view the surveyed area in Terrain Modelling (DTM). We also can animate the terrain model and view from any angle or direction. We also can prepare slices of the elevation model by copying the grid data of only the required area and then saving it in a fresh file of Excel work book. Now this new file has to be processed in the said manner for preparing the grid file.

 

 

 

This grid file will show a slice of the elevation profile of the site. Once, the terrain modelling is available, the software also allows us the examine the model in different lighting conditions. This is possible by adjusting the position of the sun in the animation. This is very useful in bringing out the subtle changes in the surface elevation of the site.

 

Web links

  • http://www.icsm.gov.au/mapping/surveying2.html
  • https://www.e-education.psu.edu/geog160/node/1926
  • https://en.wikipedia.org/wiki/Surveying
  • http://www.powershow.com/view/408e11-
    NDI5Y/Digital_Elevation_Models_DEM_Digital_Terrain_Models_DTM_Brief_Review_powerpoint_ppt
    _presentation
  • https://www.youtube.com/watch?v=1qDQNVemX4g
  • https://www.youtube.com/watch?v=4i_6eToM3X8
  • https://www.youtube.com/watch?v=9QB94t8j1Rs

 

Bibliography

  • A.H.A. Hogg, Surveying for Archaeologists and other Fieldworkers, Croom Helm, London, 1980.
  • David Clark, Plane and Geodetic Surveying for Engineers, Vol.I, CBS Publishers and Distributors, Delhi,
    2004.
  • Rudiger Mach and Peter Petschek, Visualization of Digital Terrain and Landscape DataaA Manual,
    Springer, Zurich, 2006.