Unit G1 - Resource Inventories and Mapping


Unit G1


One of the first steps in managing something, whether a resource or anything else, is finding out where it is and how much there is. You cannot manage your pigs or chickens without knowing how many you have and of what age and sex, nor can you manage a bank account without knowing how much money is in it. In the same way, you must have some evaluation of an environmental resource in order to manage it. In a forest plot, you need to know how many trees there are, and of what kinds and ages. For an agricultural area, it is necessary to measure how much there is of each soil type, slope and exposure, and what these areas are capable of producing. Since water is often limiting, it helps to know how much is available and at what times of year. A fisherman would like to know how many fish there are if he wants to fish the resource sustainably. These measures do not have to be exact, but they should be close enough to give an estimate of how much is available and how it might be changing with time.

For many resources, the first step is to determine the area covered by the resource, usually by making a map of the area with its boundaries. A map is like a drawing or picture of the land taken from high in the air. It usually shows the coastline, mountains, rivers, roads, villages and other features of the land. It can also show resources such as forests, plantations and coral reefs. You can put many kinds of information on a map, such as what kinds of crops are grown in each field, how many cattle there are in each pasture, and where the best fishing areas are. This is one way of making an inventory, which is a quantitative list or description of your resources.

A map is usually made to scale. This means that some small measure on the map is the same as some large measure on the land. On a map of a whole country, 1 centimetre on the map might be 1 kilometre or 100 kilometres on the ground. For detailed local resource maps, an appropriate scale might be 1 centimetre on the map for 10 metres or 100 metres on the ground.

Sometimes there are existing maps available from the government or in the lands department from which the general outlines of your area can be determined. For marine areas, there may be nautical charts that give the outline of the reefs and lagoons. Aerial photographs or satellite imagery may also be available, perhaps in some government department, from which the outlines of the areas can be traced. If it is possible to make at least a base map in this way, it will be easier to note the areas or boundaries of particular resources on it.

If no maps of your area are available, or if you need a more detailed map of certain places, then it will be necessary to make a simple one using one of the following methods.

Sketch map

A simple rough sketch map can be made using only a pencil, a ruler or other very straight piece of wood or metal with regular markings on it, and a piece of paper on a board or other drawing surface that is flat and hard enough to write on.

Look over the area to be mapped and decide what points, places or objects you need to locate on the map in order to draw in the important boundaries and features. These may be roads, rivers, buildings, boundary markers, large trees, the limits of watersheds, and other points of reference. Then choose the place where you are going to start making the map. This first point on the map is marked and labelled, and the board is set on the ground there. The ruler is laid on the board crossing through the first point. It is aimed at the second point by sighting along it, and a line is drawn. The distance from the first to the second point is then paced off, using steps of the same length and counting the steps. (Try pacing the same line several times to make sure your steps are the same length and the counts agree. This may take some practice.) The number of paces is written along the line, and the length from the first to the second point is marked on the line at an appropriate scale. For example, 260 paces could be 13 centimetres, for a scale of 20 paces per centimetre.

The board is then taken to the second point, laid on the ground, and oriented by sighting along the line drawn on the paper with the help of the ruler so that the line is lined up pointing back to the first point. Holding the board carefully so that it does not move, the ruler is then placed so that it crosses through the second point and is sighted at the third point. A line is then drawn, paced and measured as before, and the third point is marked and labelled.

At each corner or point along the boundary to be mapped, the board is placed on the ground and the same procedure is followed. If there is some error in closing the boundary when returning to the first point, it can usually be adjusted in redrawing the map.

If the area to be mapped is very large, it is best to subdivide it into smaller sections, map them separately, and then join the pieces of the map together. Other features to be added to the map that are not on the boundary lines can be added by sighting to them and pacing off the distance. If they are far off the line, it is best to sight and pace from two points on the line, forming a triangle, at the point of which the feature is marked.

Plane table survey

If a more accurate map is needed, then it will be necessary to work with a flat (plane) table or board with a sheet of paper attached to it, a good ruler or alidade (a ruler with sights), and a measuring tape or other means of measuring.

The plane table should be sturdy so that it will not move during the survey, and it will need to be as horizontal as possible (a carpenter's level can help with this). A thick ruler or a ruler with sights (alidade) makes it easier to sight on points above or below the horizontal. The ruler needs some kind of regular markings or graduations so that you can scale the measurements. For measuring distances along the ground, a long tape measure of perhaps 30 metres is ideal. It can be laid end to end for longer distances. If a tape measure is not available, a rope (but not one that stretches like nylon) or a chain of known length can be used. You can even make a measuring rope yourself by taking a long piece of rope or heavy cord and tying knots at regular intervals of say 5 metres. Pacing can also be used if no alternative is available, but the map will not be as accurate.

There are three methods of mapping with a plane table. The first is similar to the sketch map method shown above, with the table being moved from point to point around the boundary to be mapped. The improved equipment brings some improvement in accuracy.

In the second method, the table is placed either within or outside the area to be mapped at a point where all parts of the area can be seen. The position of the table is marked at an appropriate place on the paper, and lines of sight are drawn to each point to be mapped. It may be necessary to put a pole or stick at these points to make them easier to sight on. The distance from the plane table to each of these points is then measured with the tape or rope and marked with the ruler at an appropriate scale on the lines of sight drawn on the paper. The boundaries and labels are then added to make the final map.

The third method requires placing the table at two locations some distance apart. It must be possible to see all the points to be mapped, including the other location of the table, from both of these locations. The only measurement necessary is the distance between the two locations of the table, which should be shown by a line drawn to scale on the paper. This line is used to position the table at each location by sighting along it to the other location. While the table is at the first location with the baseline properly oriented, lines of sight are drawn toward each point to be mapped. These lines do not have to be measured or drawn to scale. The table is then moved to the other location and oriented by sighting back to the first location along the baseline. Lines of sight are again drawn to each of the points to be mapped. The places where the two lines of sight cross are the positions of the points on the map at the scale of the baseline. Be sure to label these points before moving the table.

Once the positions of the major reference points have been located on the map, it should not be too difficult to draw in boundaries, important landmarks, and detailed features, and to add appropriate labels to make the final map. The sight lines used to prepare the map can be erased when they are no longer needed, or the map can be recopied in its final form.

These mapping techniques can be used for areas that are reasonably flat and of moderate size. The larger or more irregular an area is, the more points will be needed to map it properly; it may also be necessary to subdivide such areas and map the different parts separately. Simple mapping techniques are not practical for very large areas with distances too great to measure, or for forested or hilly areas where it is not possible to see the boundaries. For such areas, using an existing map and then adding local details may be the only solution.

Using maps for resource inventories

A map of your lands and waters will be very useful for planning how to use or manage your resources. By showing the position of each resource, it may help to show where to plan for development, and where to leave resources protected or undisturbed. Since a map is drawn to scale, it is possible to measure areas on the map and then multiply by the scale to get the actual area of each resource. You can thus calculate the total area covered by forest, pastures or food crops, or the area of fishing resources.

Measurements from a map are enough to evaluate some resources like the amount of agricultural land or the area of a water catchment. For other types of resources, the area covered may need to be combined with some other measure. The size of a food crop depends not only on the area planted but also on how closely the plants are spaced. A count of a sample of the area may make it possible to estimate the resource for the whole area. For instance, suppose your map shows a large field 100 paces wide and 200 paces long planted in cassava, and you want to know how many cassavaa are growing in the field. It would take too long to count every plant, but if the cassava seem to be planted at the same density all over the field, you could count just a sample or small part of it. You would pace off a square of perhaps 10 paces by 10 paces (10 x 10 = 100 square paces) and count all the cassava in the square. Since there are 100 x 200 = 20,000 square paces in the field and your sample is one two hundredth of this, you would multiply your count by 200 to get an estimate of the total number of cassava. If you counted 120 plants in your sample square, there would be about 24,000 cassava in the whole field. The same procedure can be used for forest trees, reef fish, or other resources.

Other inventory techniques

Not all resources can be evaluated on a map or in relation to a certain area, but they can sometimes be estimated along a line of known length, or over a set period of time. For instance, forest birds can be estimated by counting the number seen or heard while walking along say 1 kilometre of forest path at dawn, and trying to avoid counting the same bird more than once. Fish can be counted while swimming along a known length of reef front. Mosquitoes can be evaluated by counting the number that come to bite you on the arm while sitting still for 15 minutes at nightfall. While these methods may not give total numbers, they can be repeated in exactly the same way to measure changes over time, such as the effects of heavy fishing or of spraying for mosquito control.

Estimates for a limited area can often be projected to the whole larger area of interest to give a value for the whole resource, as was done for the sample of the cassava field above. Suppose that your community has 15 kilometres of shoreline with a fringing reef, and that there is no reason to think that there will be more fish on one part of the reef than another. If you have swum several times along 100 metres of reef edge, and each time you counted at least 3 large fish, then you can estimate the total number of such fish along the 15 km (15,000 metres) of reef at 3 x 150 = 450 fish. Knowing this, the community could plan on catching perhaps 50 to 90 such fish during the year and still leave a population able to reproduce and grow to a reasonable size (say 5 years old). The simple inventory has permitted a first step towards management. If you repeat the survey a year later and find that there are now fewer fish, then the community catch level may be too high and may need to be reduced. This repeated measurement is monitoring of the resource to see how it may be changing.

The same type of inventories can be made for valuable trees in a forest, or for garden areas planted or resting in fallow. Making a complete inventory of the resources available to a community can help to suggest where development can take place and where resources are already being used to their limit, or even being pushed past their limit towards exhaustion.


What is the first thing you need to do in order to manage a resource?

What is a map? What does it show?

Are there already maps available for your area? If not, do you think it would be useful to make one?

Do you need complicated equipment to make a map? What do you need?

What kinds of resources can be shown on a map?

What resources would it be useful to map in your area?

What are some other techniques that can be used to estimate resources?

Why is it useful to repeat an inventory exactly the same way sometime later?

Instructions for trainers in the use of this unit

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Last updated 21 June 2008