Notes - Archaeological Use of a Compass
The following are updated notes used in discussing compass use in a field methods course given at the University of Western Ontario.
Advantages and Disadvantages of a Compass Over a Transit
- The accuracy of the compass is less - It is accurate to one degree if used carefully. An error of one degree will lead to and error of one unit in sixty. (e.g. 2m error over a distance of 120m).
- Distance is not measured by the compass.
- A compass does not incidentally record elevation which could be used for a contour map.
- A good compass will sometimes include as second measurement device which can be used
to derive vertical distance but it is difficult and you still need the distance. This is useful to get the height of a ridge for example.
- The compass is highly portability. This is very useful when doing site survey.
- You do not need help to take a set of readings. One person can do it all.
- A single pair of compass readings, for example locational information about a site (see Figure 2) can be taken in under a minute whereas transit setup and reading would take 10 - 15 minutes and still be only as accurate as the compass.
- Both can be used to do a controlled surface pickup (mapping the relative locations
of surface finds on a site).
How to Use and Read the Compass
- A compass being used for survey or mapping should be capable of readings accurate to
one degree. The compass should have one mark per degree and a bubble level for horizontal use.
It is also required that you be able to move your eye from the level to the alignment indicators
and then to the direction reading with no body movement since you will not be setting the thing up on a tripod.
- Aligning the compass can be tricky. Use a cyclical process. First site in the datum point, then level the compass. Repeat this process until the compass is level and pointed at the datum point. When both of these are OK, move your eyes over to take the reading. Do not move the compass
- There are two ends to the compass so always read the direction "from" the
datum point. This will provide consistency and gives the same direction as if a transit were
set up at the datum point. However, if you do read the wrong end, the calculations to create
Cartesian coordinates will still work. It may look a little odd though if you are plotting
Selecting Datum Points
- Selection of these is critical because it is frequently necessary to return to the site and reestablish the original datum points.
- You can use trees, fence posts or buildings.
- The problem occurs because landscape changes. Trees are cut, fences are removed etc.
- Iron bars are good but even these can be moved or the location can be lost. Typically they are left flush with the ground so that even a year later they can be difficult to locate unless you can remember almost exactly where it was. A metal detector can be useful for locating lost iron bars.
- You should be specific enough that someone else could follow your directions and relocate the datum points.
- Select starting points that will be identifiable on a top map or an aerial photograph if at all possible.
- You should capture the distance and direction between the datum points the same day you first use them. You may not get back to measure them or they might change.
- If natural features are not plentiful, another technique is to establish a series of stakes at 30m (or whatever) intervals. Figure 3 was done this way.
- Another technique to locate a datum point can be the distance down a surveyed line (property boundary) from the centre line of a road.
- Once you have selected a datum point, it makes things much easier to take readings if you either mark it with fluorescent orange or put up a bright marker unless the point is clearly demarcated against the background.
Mapping - Compass and Pace
- A full description of this can be found in most field methods books.
- This is really the same approach as using a transit - you record distance and direction.
- You need to take directions from fixed points and then pace out the distance.
- Accuracy of a "pace" can be questionable. It would be best to use a normal pace and then count these over a known distance several times to work out your own approximate pace distance.
- This can be labour intensive in that you need to pace everything off.
- This might be useful for locating single interesting find spots or site locations but would be unworkable for a CSP.
Mapping - Compass and Tape
- This involves similar tactics to "Compass and Pace" but more accurate with respect to distance.
In using either compass and pace or compass and tape methods, the readings are converted to Cartesian coordinates using the same caluclation as for transit data.
Mapping - Compass Only - Triangulation
- To start, establish a series of datum points. If at all possible, these should be able to be oriented on a top map or aerial photograph.
- Next record the distance and the direction between the datum points.
- When recording locations relative to the established datum pointds, take two readings, one from each of two distinct datum points. Figure 1 shows how this would be done.
- With this information you can construct a map showing the datum points and each recorded find spot. See Figure 3 for an example.
- Watch the intersection of the two readings. Under 30 degrees and over 150 degrees result in lower accuracy.
- You should also record the locations of prominent landscape features such as edge of field, trees, fence lines, roads, water courses etc. These will be required to draw the final site map or to orient the CSP on an aerial photograph.
Mapping - Creating the Map Back in the Lab
- When you need to produce a map, one option is to plot to scale using a protractor which shows 360 degrees and a ruler. Air navigation protractors can be purchased at any flight school and are ideal.
- Another alternative is to feed all the readings into a computer and then automate the map production. Spreadsheets to accomplish this have been published on this web site so no mathematical skill is required. Autocad, Surfer or any GIS software can plot your locations. Figure 3 was done with MFWorks a GIS system from Thinkspace.
- Another tactic is to plot sites or artefacts onto either large scale top maps or aerial photographs. Figure 2 plots a site location on a scanned portion of a topographic map.
- Aerial photographs can be used to plot sites or artefacts. You will need to select one or two points that will show on the photograph.
- In using aerial photographs sections of large scale top maps, you will need to orient the base map or photograph as to direction. Getting the direction of a road or fence line close by is ideal. In figure 2 the road was used.
- Multiple artefacts can be measured from the same location. However, you need to reflect the number of artefacts on the final map. Use a randomizing algorithm. You should end up with one dot per artefact to get a proper pictorial representation of the density. Figure 3 uses a GIS function to create multiple dots but this is not ideal as it uses a recurring pattern rather than a randomizing approach which can be seen looking closely at Figure 3.
Figure 2: Sample Top Map use. In the following example a site is located by taking
two readings from locations which are defined on the original top map. The top map is derived from a 1/10,
000 map that was scanned and blown up. In this case, as are we are not plotting on a top map
where magnetic north can be precisely determined, it is also necessary to take a reading in the
field so that the plotting compass can be properly oriented back in the lab. In this case the
direction of the road to the south was taken. In order to place the site on the map, first
place the compass on the road so that the measured direction is indicated and then draw one or
more lines pointing to magnetic north. The green lines were plotted this way.
Finally place the compass over each of the two datum points, orient it to the north and mark
each of the recorded directions (the red lines). The intersection shows the site location.
Figure 3: Sample CSP created with a compass. The next example shows a plot of a CSP
which was done with a compass. The datum points were stakes place every 30m along the north
edge of the feild. The readings were then inserted into a database and run through a program
which calculates Cartesian coordinates using a similar calculation to the spreadsheet provided
on the previous page.
The X-Y coordinates were then imported into MFWorks, a GIS system, which
plotted the map as shown.
Figure 4 CSP overlaid on and aerial photograph The CSP from figure 3 has been placed
on an aerial photograph and the site boundary has been defined. MFWorks was used again.