How to plot from aerial photographs onto maps

Sketch plotting

This method needs a good eye to work out the scale of the photograph to match it to the scale of the map you want to plot onto. This method should be used to plot onto maps at 1:10,000 scale that can be printed off from the Unlocking Buckinghamshire’s Past website.

 

With this method, you have to look at the field boundaries around the archaeological feature you have noticed on your aerial photograph. Line up the field boundaries on your photograph with those on the map. Fields are usually very distinctive. Any other features can be used to guide your plot, like roads, pathways, electricity pylons and houses.

 

When you think you know where your archaeological feature is on the map, think about what size your features are in relation to the size of the field and any other features on your photograph. When you are happy with the size of your feature, plot it on your map!

Network plotting

This method gives more accurate results. You can plot onto maps at smaller scales, such as 1:5,000. It consists of drawing lines onto the map and the photograph between corners of the field. These lines have to cross the features you want to plot at some point.

 

When you have matched the field boundaries on the aerial photograph with those on a map, as in the first step in sketch plotting, selotape some acetate paper onto the photograph (you don’t want to draw directly onto the photograph). Draw three or four lines with a ruler between distinctive features onto your map, such as between the corners of the field you want to plot onto, or between an electricity pylon and where a pathway enters the field and so on. Try to pick these lines so that they cross the features you can see on the aerial photograph. Measure the length of your line. Convert this to the length it represents on your map. Write these on a separate piece of paper. For instance, a three centimetre line may represent 300 metres.

 

Once you have drawn the lines on the map, draw the same lines onto the acetate over your aerial photograph. Because the aerial photograph has been taken at an angle rather than vertically, the lines will probably be shorter. Write the lengths of each corresponding line on your sheet of paper. For instance, if the three centimetre line is only two centimetres on your aerial photograph, write that down. This will act as a guide to help you remember the scale.

 

The next step is to measure the line on your aerial photograph between one of your distinctive features to where it crosses the archaeological feature you can see. If it is one centimetre, for instance, then that corresponds to 1.5 cm on the map (because 1 is half of 2, 1.5 is half of 3). This also means that, on the ground, the feature is 150m along your line. For more complicated measurements here is the formula:

 

Work out the percentage of the measurement along the line on the photograph. For instance, if the whole line is 2 cm and the measurement to the archaeological feature is 1cm, then divide 1 by 2. If you do this on a calculator you get 0.5. This means 50% (always move the decimal point two places to the right to convert decimals to percentages). Next, divide the measurement of the whole of the same line on your map by 100. So if the line on the map is 3 cm long, divide it by 100 and you get 0.03 cm (this is 1% of the line). Multiply this by the percentage you worked out before, in this case 50, and you get 1.5 cm. Mark this on your map. You need to take the measurements wherever your lines cross the archaeological features and convert the length until you have a number of points marked on your map.

 

Print off the next two pictures as large as possible. One is an aerial photograph of Cheddington Hillfort with four lines of different colours on it. The other is the map of the same area with the corresponding lines. You will first notice that the photograph was taken from the north looking south but the map, of course, has north at the top.

 

Please contact the County Archaeological Service for the modern map of the Cheddington hillfort area.

 

Measure each line on both the aerial photograph and the map. Write the measurements here.

	Aerial photograph (cm)	Map (cm)
Blue
Red
Green
Yellow

Now take measurements of each of the following segments of line:

Blue (cm)	Red (cm)	Green (cm)	Yellow (cm)
a =	d =	g =	j =
b =	e =	h =	k =
c =	f =	i =	l =

Divide a, b and c by the complete length of the blue line; divide d, e and f by the complete length of the red line and so on. Write down in this table below the result you get to the accuracy of two decimal places and convert it to a percentage by moving the decimal point two places to the right.

Calculation	Result to 2 decimal places	Percentage
a ÷ blue line
b ÷ blue line
c ÷ blue line
d ÷ red line
e ÷ red line
f ÷ red line
g ÷ green line
h ÷ green line
i ÷ green line
j ÷ yellow line
k ÷ yellow line
l ÷ yellow line

Before you go any further, it is useful to know how long 1% of each line on the map is, as you will have to multiply it by all the results from the table above. To get the length of 1% of your line simply divide the length of the line by 100. Put the result to the accuracy of two decimal places in the column on the right.

Map line length	Length of 1% of the line

Now you are ready to work out where to start putting your marks on the map. Multiply the percentage of each segment by the length of 1% of the line on the map:

Segment	Percentage	1% of line on map	Length of segment on map
a
b
c
d
e
f
g
h
i
j
k
l

Now you are ready to start marking points. Mark out the segments on the lines on the map. When you have the points marked, you can join the dots!

 

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