FAO STATISTICAL DEVELOPMENT SERIES
CONDUCTING AGRICULTURAL CENSUSES AND SURVEYS
FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS
5.1 Maps are used to produce more accurate, timely and useful statistical data. Maps are essential in planning the agricultural survey or census, for data collection, and for the presentation and analysis of results.
5.2 Maps of several types are used for agricultural census or survey purposes, that is maps related to the agricultural characteristics: topographic charts, cadastral maps, road maps, administrative area maps, maps showing the population distribution, aerial photographs, satellite images, space photographs, land use, soil or geologic maps, etc. Sketches are also sometimes used to help delineate the enumerators' areas of work.
5.3 This chapter deals with cartographic materials which are usually maps or sketches printed on paper. The attention of the reader should also be drawn to other kinds of support for maps, such as tapes, diskettes, CD-roms, which facilitate the use of maps, allowing changes of scales (see Frame 5.1 Scale on maps), geometric corrections, juxtaposition or superposition of thematic maps. Such maps on magnetic media constitute the basic components of Geographic Information Systems (GIS), which require the availability of experienced staff, powerful computers and software (see Chapter 6).
5.4 A pre-requisite for the use of maps is a comprehensive inventory of available maps that could be used for agricultural statistical purposes. In fact, the main task in cartographic preparations for an agricultural survey generally consists of adapting, revising and updating available maps to the survey requirements.
The most important parameter on a map is the scale, which is the ratio of representation of the magnitude of the land. The scale must always be present on a map and usually appears as a fraction spreading between 1/5,000 (large scale) to 1/1,000,000 or less (small scale).
1/5,000 means that 1 unit of length on the map represents 5,000 units of length on earth, i.e. 1 cm= 50m.
Large scales are used to represent small areas of earth; for example on an A4 sheet (21x29.7cm) one could place in width:
- 1 km at the scale 1/5,000
- but 200 km at the scale 1/1,000,000.
In other words, a country of 1,000kmx1,000km needs at least paper measuring 1mx1m to be totally represented and the scale cannot be larger than 1/1,000,000.
Thus, the choice of the scale depends on the purpose for which the map is to be used but attention should also be paid to paper size and, therefore, the manageability of the document, especially for field work.
5.5 A large proportion of the cartographic preparations for an agricultural census or survey consists of delineating and identifying the enumerators' areas of work, i.e., the enumeration areas (EAs) which are components of the agricultural census frame. Theconstruction of the census frame should therefore be undertaken as part of the census field data collection preparatory activities (see Chapter 6).
5.7 The largest proportion of the cartographic support for an agricultural sample survey is the preparation of the sampling frames corresponding to each sample selection stage, including detailed mapping to support the field data collection.
Purpose of maps
5.8 Maps for conducting agricultural censuses and surveys have three main purposes:
- 5.8.1Planning and supervising operations.
- These are usually small-scale topographic maps, say 1/50,000, 1/100,000 or smaller, showing political and administrative boundaries, location of cities, towns and villages, mountains, plains, lowlands, valleys, rivers, deserts, swamps, transportation lines, and some indication of population density or extent of agricultural areas. They may include data on vegetation, land use or land cover features, and may be satellite images. They should provide the cartographic base for planning and organizing the statistical work, for instance to set up enumerators' assignments, assign geographic codes to proper statistical areas, estimate travel distances and directions, etc. Maps used for the construction of the frames are also included in this category of maps. For a sample survey, a sample frame is needed for each stage of sample selection, and therefore the appropriate supporting maps are generally of a larger scale than those previously mentioned.
- 5.8.2Field Data Collection.
- Maps (or sketches) and/or photos for identifying enumeration areas (EAs) are used in the field by census/survey enumerators for data collection. These are large-scale field maps, 1/10,000, 1/5,000 or larger, which should help the enumerator locate the agricultural holders or holdings' addresses to assure complete coverage of areas without omission and duplication, determine the best route of travel to and within the enumeration area, measure distances, determine directions, show the progress of the field work and, ideally, to identify the agricultural holdings (land and even buildings, if possible). Enumeration area map symbols should follow cartographic standards and should preferably be self-explanatory, not requiring special complex training instructions for the enumerators. In any case, the use of maps should be an essential part of the enumerators' training.
- 5.8.3Presentation and analysis of results.
- Maps can be used to relate statistical data with the corresponding geographic area, facilitate the understanding of statistics and assure a more extended and appropriate use of data. Maps provide a means by which statistical information can be presented simply andeffectively. Atlases produced from the statistical results, are often useful publications (see Chapter 18).
Exploration of existing map resources inventory
5.9 One of the first activities in the planning of an agricultural census or survey should be to explore in detail the feasibility of using existing maps. It is strongly recommended to prepare an inventory of existing maps including, at least, the following information:
- Office responsible for the preparation.
- Date of publication or preparation.
- Date of the basic photography if made from aerial photos.
- Purpose of the preparation.
- Area covered.
- Cartographic references.
- Technology used.
The map inventory should then be evaluated for census or survey use by cartographers.
Types of maps used
5.10 Many different types of maps can be used for the agricultural census or survey, for example:
- 5.10.1Topographic sheets.
- The most important maps available in various government branches are general topographic maps that are published in sections called topographic sheets. Most European topographic sheets are on the scale of 1/25,000 to 1/100,000. Less-developed countries generally use smaller scales: 1:100,000 or smaller.
- 5.10.2Other government maps.
- Maps may be available from government offices involved in land surveys. For example, geological survey maps, coast and geodetic survey maps, topographic and hydrographic survey maps, land-use and land cover maps, conservation and land reclamation maps, armed forces maps, forest and wildlife maps, etc.
- 5.10.3Satellite images.
- Satellite images (available on paper or on magnetic media) are very valuable and provide useful information as they give a detailed and up-to-date picture of the land and can provide information on land use, agricultural patterns and practices, population density and infrastructures. Satellite images are used in a number of countries, for example as an instrument for improving the methods of agricultural statistics data collection. More precisely, satellite images are used for the following purposes:
- to identify and subdivide the agricultural land (stratify) by intensity of land use and other land cover characteristics and therefore help in the construction of area sampling frames for agricultural surveys; and
- to monitor agricultural changes by using, for instance, vegetation indexes.
- 5.10.4Aerial photography.
- Aerial photos can be used to produce, update and supplement census maps. If no map exists for an area, or if the ones available are seriously out-of-date, it may cost less to take aerial photographs than to construct maps. On the other hand, if there is a shortage of resources for taking the photographs and interpreting them, drawing sketch maps may be the best solution. Aerial photos are commonly used for data collection of selected areas (segments) in area sample surveys, as indicated in Chapter 6.
- 5.10.5Communication maps.
- All forms of transportation, land, sea, and air, need maps to show their routes to the public. Quite often such maps are diagrammatic, that is the presentation is very simple. Some of these maps are totally utilitarian in nature as in the case of railroad maps. However, many are good landscape maps, showing patterns of vegetation, types of farming, etc. Maps prepared by airline companies and some road maps usually include scenic and places of historical interest, recreational spots and other information to promote travel.
- 5.10.6Land-use maps.
- Such maps show the actual and possible uses of land, including both agricultural and non-agricultural (industrial, urban, recreational, mining and lumbering).
- 5.10.7Economic maps.
- These maps are concerned with the production, transportation and distribution of goods, and are necessarily small-scale maps. Economic maps are likely to focus on a single product or a group of products and are mostly statistical in nature.
- 5.10.8City and tourist maps.
- Although such maps are primarily intended to motivate tourism, their utility for census purposes should not be overlooked. Besides tourist attractions, such maps also show the road systems of cities and other tourist areas.
Timing of cartographic preparation
5.11 The timing of cartographic preparation is very important. Statistical offices should try to carry out cartographic work on a permanent basis in order to continuously update the statistical maps. This requires permanent specialized staff, which could be increased prior to and immediately following census and survey activities for the updating of maps with the necessary information.
5.12 The timing of the cartographic preparations naturally depends on the type of statistical operation, the desired geographic precision required for data collection and dissemination, the availability and accuracy of the basic cartography of the country, the number of maps needed, and other characteristics of the country like topography and land use.
5.13 To give an example, it should be mentioned that some large countries in Latin America have devoted no less than two years to census cartographic preparation.
5.14 Computer-assisted cartographic systems can be very useful to continuously update maps for statistical purposes.
U.S. Bureau of the Census. Mapping for Censuses and Surveys, Statistical Training Document, ISP-TR-3, Washington, D.C., 1978
U.S. Bureau of the Census. Mapping for Censuses and Surveys, Workbook, Statistical Training Document, ISP-TR-3W, Washington, D.C., 1978