Mapping rural/urban areas from population density grids F.J. Gallego, Institute for Environment and Sustainability, JRC, Ispra (Italy) Summary We discuss several possible criteria to classify the European Union (EU) into rural and urban areas. Some unexpected results are highlighted with definitions based only on population density per administrative unit or only on land cover type. These abnormal results come partly because the definitions depend too strongly on the size of the communal territory. In the approach we propose as a basis for discussion, the classification units are the communes, but a major role is played by urban agglomerations, that are defined and determined in a way that does not depend on administrative boundaries. The classification proposed has three major groups: urban, semiurban and rural. Each of them has been subdivided, but different criteria for subclassification have been prepared. Some definitions of rural and urban areas For the definition of a policy on rural areas development, a logical previous step is defining which areas are rural and which areas are urban. The issue is not trivial, as we can think at first sight. There is no doubt that the centre of Paris is urban or a remote area in Lapland is rural, but there are many intermediate situations and it is difficult to give a definition that is objective, practical, applicable across the European Union (EU25), and that takes into account the different aspects of rurality. One of the issues to be considered is the choice of basic units for the classification. Homogeneous areas can be defined by clustering small basic units (Geddes and Flowerdew, 2000); one option is starting with cells of a regular grid (Librecht et al., 2004). However for small units, few data are available, and often less reliable. For the approach given here, the commune is selected as basic unit, because it seems a realistic choice for policy application. The concept of “rural area” involves a number of socioeconomic aspects, such as structure of the employment, population age population change. Unfortunately these data are difficult to collect at the commune level for EU25, and we have based the study on population density and land cover. Mapping rural/urban areas from population density grids 1 OECD definition. The OECD has given a definition of rural areas based on the percentage of the population of a region living in rural communes (OECD, 1994). A commune is classified as rural if the population density is below 150 inhabitants per km2. This definition has the merit of being easily applicable, but has some limitations: • The commune classification into urban/rural depends too much on the area of the commune. Let us put an example to illustrate some awkward effects of this definition: In Extremadura (Spain), this definition gives only 6 urban communes, out of which 4 have less than 6,000 inhabitants. The main cities of the region (Badajoz, Cáceres and Mérida) are classified as rural because their communes include wide areas of woodland and shrub. • It does not take into account the characteristics of the surrounding area, in particular if it belongs to the outskirts of a big city. Figure 1 shows the case of Aldea de Trujillo, classified as urban because the communal territory only includes the small urban nucleus (439 inhabitants in 0.3 km2), without taking into account that it is surrounded by natural and agricultural areas. • Some relatively large towns are labelled as rural because the communal territory contains large “empty” areas (table 1). More than 250 communes above 20,000 inhabitants have a density < 150 and would be considered as rural with the OECD definition. Most of them have a fairly large urban nucleus. If the commune is used as classification unit, a specific category might be meaningful for this type of communes containing an urban nucleus and large agricultural or natural areas. Population Aldea de Trujillo Calamonte Puebla de la calzada Valle de Santa Ana 439 5564 5480 1338 Area commune (ha) 35 770 1419 376 Density 1272 723 386 356 Table 1. Communes with the highest population density in Extremadura Population Badajoz Cáceres Mérida 122225 74589 49284 Area commune 153434 175139 86785 Density 80 43 57 Table 2. Main communes in Extremadura (classified as rural) Mapping rural/urban areas from population density grids 2 Figure 1: Example of small village (Aldea de Trujillo) classified as urban because the communal territory is small. Table 3: largest towns with density < 150 inhab/km2 Jerez de la Frontera Uppsala Albacete Linkoeping Badajoz Oerebro Norrkoeping Vaesteraas Joenkoeping Boraas Area commune (ha) 141180 246459 124310 143086 153080 184006 149073 95643 148512 118039 Population commune 183316 167508 130023 122268 122225 120944 120522 119761 111486 101766 density 130 68 105 85 80 66 81 125 75 86 Rural and Urban regions (NUTS3 units) On this basis the rural/urban classification of communes with a density threshold of 150 inhab/km2, the OECD definition distinguishes three main categories of regions: • mainly rural regions: more than 50% of the region's population live in rural communes; • relatively rural regions: between 15% and 50% of the population lives in rural communes; Mapping rural/urban areas from population density grids 3 • mainly urban regions: less than 15% of the region's population lives in rural communes. Figure 2: Rural and urban NUTS3 units according to OECD definition. (Scotland, East Germany and islands missing). The criterion is reasonable but has some limitations and produce unexpected results that depend on the delimitation of communal boundaries, in particular for large, heterogeneous NUTS3 units, for which attributing the same label of rurality to the whole NUTS3 unit may be unfair. For example the only province in Sicily that turns out to be “mainly urban” is Ragusa, while Stockholm is relatively rural. Land cover in Rural/Urban communes with the OECD definition. Mapping rural/urban areas from population density grids 4 An interesting check for the delimitation of rural/urban areas is the proportion of agricultural land. In principle we would expect that urban areas have a low proportion of agricultural or arable land. We can estimate such proportion with the help of the European point survey LUCAS, -Land Use/Cover Area-frame Survey- (Bettio et al, 2002). The definition based on the population density for the commune gives an unexpected result: the average proportion of arable land in communes with a population density above 150 inh/km2 is 29%, versus 21% in communes with less than 150 inh/km2 (table 4). For Utilised Agricultural Area (UAA), the proportion is 48% in “urban” communes versus 40% in “rural” communes. The same question can be put for NUTS3 units: which is the percentage of arable land (or agricultural land) in NUTS3 regions labelled as mainly urban, relatively rural or mainly rural. Tables 6 and 7 give an estimate of such proportions using LUCAS data. Rural (< 150 inh/km2) LUCAS SSU % arable total arable AT BE DE (West) DK ES FI FR GR IE IT LU NL PT SE UK (No Scotland) total 363 109 1404 715 2973 705 4839 743 96 1672 15 209 551 762 1135 16291 2310 443 4421 1152 11727 10216 15285 3759 2090 6727 67 398 2404 12902 3910 77811 16 25 32 62 25 7 32 20 5 25 22 53 23 6 29 21 Urban (> 150 inh/km2) LUCAS SSU % arable total arable 29 136 763 72 235 11 429 30 5 811 7 398 70 37 265 3298 217 520 2906 183 886 157 1614 124 70 2462 13 737 313 114 1155 11471 13 26 26 39 27 7 27 24 7 33 54 54 22 32 23 29 Table 4 : % of arable land in communes with population density above/below 150 inhab/km2 Mapping rural/urban areas from population density grids 5 AT BE DE (West) DK ES FI FR GR IE IT LU NL PT SE UK (No Scotland) total Rural (< 150 inh/km2) LUCAS SSU % UAA total agricultural Urban (> 150 inh/km2) LUCAS SSU % UAA total agricultural 762 242 2471 794 5921 725 8813 1664 1597 3115 46 229 864 1004 2981 31228 64 258 1329 83 405 12 686 64 41 1424 10 420 105 41 547 5489 2310 443 4421 1152 11727 10216 15285 3759 2090 6727 67 398 2404 12902 3910 77811 33 55 56 69 50 7 58 44 76 46 69 58 36 8 76 40 217 520 2906 183 886 157 1614 124 70 2462 13 737 313 114 1155 11471 29 50 46 45 46 8 43 52 59 58 77 57 34 36 47 48 Table 5: % of area used for agriculture (UAA) in communes with population density above/below 150 inhab/km2 AT BE DE DK ES FI FR GR IE IT LU NL PT SE UK Total Mainly urban 19 55 38 12 25 n.a 46 36 n.a 46 n.a 56 25 n.a 57 45 NUTS3 Relatively rural 28 55 55 57 49 19 57 61 75 51 70 57 24 33 75 54 Mainly rural 34 40 55 71 55 6 56 43 76 49 n.a 69 40 7 89 33 average 33 52 52 65 50 7 56 44 76 49 70 56 36 8 65 42 Table 6: % of the territory used for agriculture (UAA) in rural/urban NUTS3 with the OECD definition Mapping rural/urban areas from population density grids 6 AT BE DE DK ES FI FR GR IE IT LU NL PT SE UK Total Mainly urban 12 31 22 12 10 n.a 28 4 n.a 25 n.a 53 19 n.a 24 25 NUTS3 Relatively rural 14 30 31 49 26 19 33 23 10 28 28 53 15 27 31 29 Mainly rural 16 9 31 64 27 6 29 20 4 27 n.a 61 26 5 1 17 average 16 25 33 59 25 7 31 20 5 27 28 53 23 6 22 22 Table 7: % of the territory used for arable land in rural/urban NUTS3 Classifying rural/urban on the basis of artificial area. A method has been explored by DG Agriculture to classify communes into urban and rural on the basis of land cover (DG Agri, 2004). The area of the commune occupied by artificial land is estimated as well as the area of “rural” land cover types (agriculture, forest and natural areas). The estimation is made by simple measurement of major land cover classes in CORINE Land Cover (CLC). CLC is a land cover map that has been produced with approximately homogeneous specifications in all EU-25, except Sweden (CEC, 1993, EEA, 2001). Areas of water are excluded from this computation. A commune can be defined as urban if the artificial areas occupy more than a certain threshold (for example 10%, 30%). This type of definition is easy to apply if CORINE Land Cover is available, but may produce some unexpected results, similar in some cases to the anomalies found with the OECD definition, for example communes with a very small territory are likely to be classified as urban. The example of Aldea de Trujillo mentioned above holds again, but it is not the only one: 637 communes have been identified with more than 30% of artificial are in CLC and out of urban agglomerations of more than 5000 inhabitants (see below method to define the agglomerations). Figure 3 shows another example in Poitou-Charentes. Mapping rural/urban areas from population density grids 7 Argenton-Château (Deux-Sèvres) ~1000 inhab, 49% artificial (CLC) Figure 3: An example of small commune in Poitou-Charentes (FR) with a high % of artificial land in CLC. With this definition, a number of major cities might be classified as rural because the communal territories are large (table 8). As suggested above, this type of large communes containing a significant agricultural or natural land is considered a specific category in the draft classification suggested in this paper. Commune Roma Valencia Zaragoza Málaga Genova Helsinki Szczecin Palma de Mallorca Córdoba Valladolid Murcia Ljubljana Country IT ES ES ES IT FI PL ES ES ES ES SI Population* (1000 inh.) 3089 847 693 688 675 551 432 413 404 388 380 379 Area (km2) 1505 135 1065 396 239 685 301 209 1257 198 888 275 % artificial in CLC 27 29 6 15 25 15 30 22 6 14 4 24 Table 8: some cities with less than 30% artificial land in CLC. * Census data were not available for this work. Population data have been derived from the Landscan raster layer. These figures are not very inaccurate and should be seen as an indication of the order of magnitude. This classification can be nuanced by separating several groups of artificial land cover types. CLC nomenclature contains 11 artificial classes, and some of them, such as airports, mines, dump sites and leisure facilities, are not necessarily characteristic of an urban environment. Mapping rural/urban areas from population density grids 8 Figure 4 shows some examples of communes that do not belong to any urban agglomeration), but have a significant area for an airport, leisure zone, dump site or mines. Vintirov (CZ) 36% dump site, 15% mine 12% industrial Figure 4: Examples of communes out of urban agglomerations with a high % of artificial land cover in CLC. A GIS approach to define urban agglomerations. For policy definition purposes, the natural unit might be the commune. The characterisation of a commune should take into account the structure of population density inside the commune and in the neighbouring communes. For this purpose, it is useful to define urban agglomerations without using the administrative boundaries of communes. This can be done if suitable information is available on the population density. Population density represented in raster mode provides an ideal tool for this type of analysis. Two layers of information are actually available for this task: Mapping rural/urban areas from population density grids 9 • • Disaggregation of the 1991 census data by commune with the help of CORINE Land Cover (Gallego and Peedell, 2001). The grid should have covered EU15, but due to various problems of data availability (CLC or commune boundaries), several major areas are missing: Sweden, Finland, Scotland and the eastern Länder of Germany. The resolution of this population density grid is 1 ha in equal-area Lambert Azimuthal coordinates. Subset of the Landscan global population database (Dobson et al, 2000) in Lat-long co-ordinates. The reference date is 2002 and the resolution is 0.5’, corresponding approximately to 920 meters in the north-south direction and a varying width (750 meters at 35° latitude and 390 meters at 65°). This grid has been produced by disaggregation of regional data. The 1 ha grid obtained from 1991 commune data is more accurate, but the coverage incompleteness is a serious limitation. With this layer we have applied a series of filters to define urban agglomerations: • Smoothing by averaging values in a circle of radius 500 m. Water was excluded from this operation by masking with CLC. • Applying a threshold of 500 inh/km2. • Majority filter with a 11x11 moving window to smooth the shape and eliminate small centres (with mask). • Buffering with a 5x5 moving window (no mask). • Converting to polygon shape. • Computing the total population of each polygon. Selecting polygons > 5000 inhab. The operation was slightly different with the 2002 Landscan layer in lat-long coordinates to adapt for the coarser resolution. Steps were: • Converting the population layer (estimated number of inhabitants per cell) into a density layer dividing by the area of each cell (varying with latitude). • threshold of 500 inh/km2 • majority filter 3x3 cells (with coast line as mask) • Buffer by “maximum” in a 3x3 moving window (no mask) • Converting to polygons • Estimating the population of each urban polygon by summing the values of the grid cells inside the polygon. • Selecting nuclei > 5000 inh. (notice that the population estimated from the grid layer is not very accurate, in particular for small communes). Urban agglomerations are classified by classes of size. A total of 3886 agglomerations have been identified counting for a total of 287 million inhabitants For the small size classes, the number of nuclei identified depends very strongly on the characteristics of the population grid. A first visual inspection suggests that the population of rural centres below 20,000 inhabitants may be strongly underestimated. Further check is needed. Other abnormal results can be found. Figure 5 illustrates the case of Tampere (Finland) in which the algorithm Mapping rural/urban areas from population density grids 10 identified 2 separate agglomerations disregarding the link between them. The shape of both polygons does not perfectly correspond to the CLC urban area. Size (in 1000 inh) 5-10 10-20 20-50 50-100 100-200 200-500 500-1000 1000-2000 2000-5000 >5000 N. nuclei 1303 808 911 453 207 128 36 24 13 3 Total pop 9302 11109 29894 31789 28148 39301 26029 35848 45334 30142 Table 9 : number of nuclei identified > 5000 inhab. per size class. Figure 5: a slightly abnormal result in Finland. Figure 6 shows the different results obtained from both population layers in the Ruhr-Rhine area between Dortmund and Bonn. In Both cases the area is identified as the largest urban agglomeration in Europe, but working with the coarse resolution layer leads to a larger single polygon where the finer resolution had separated several agglomerations. This is due to several reasons: different reference date, different accuracies, different parameters chosen for smoothing and buffering, etc., but illustrates as well that a certain subjectivity remains always when urban agglomerations are defined, even if it is through a GIS algorithm. Some agglomerations have been estimated above 50.000 inhabitants from Landscan and below for the 1991 grid. Mapping rural/urban areas from population density grids 11 Figure 6: different urban agglomerations (>50.000 inh.) identified from the 1991 population grid (1 ha resolution) and the 2002 Landscan grid. Draft proposal for a typology of rural/urban communes We have classified nearly 108,000 communes into the three major categories, each of which has been subdivided and can be further split with different criteria. Each class can be subdivided by size of nucleus, by land cover profile (predominantly arable, forest…), by topographic roughness (mountain, hill, plain), by soil quality, etc. The major classes proposed are: Urban • Fully urban communes: > 99% in an urban nucleus>5,000 inhab. • Mainly urban communes with moderate rural area: 50-99% in an urban nucleus>5,000 inhab. Semi-urban • Communes with an urban nucleus and large rural area: Dominant commune of an urban nucleus (Medium-small urban centres of rural areas). A commune can be considered dominant in the nucleus if it has > 50% of the population of the nucleus, Mapping rural/urban areas from population density grids 12 • Suburban (peripherical urban): intersects an urban nucleus >5,000 inhab and is not in any of the previous categories. This category still needs further analysis. Many communes that should have been classified as rural peri-urban appear in this class because of the spatial inaccuracy delineating urban agglomerations with Landscan database. Rural • Peri-urban rural areas: does not intersect with any urban nucleus>5,000 inhab., but is in the area of influence of an urban agglomeration. The area of influence has been defined through a “gravity indicator” (Wang and Gouldmann, 1996), that takes into account the population of the agglomeration. • Remote rural: distant from urban agglomerations. Urban and semi-urban communes. We found 10999 communes with more than 50% of the territory in an urban agglomeration. 6377 were >99% inside the urban agglomeration. Size of the agglomeration (in 1000 inh) 5-10 10-20 20-50 50-100 100-200 200-500 500-1000 1000-2000 2000-5000 >5000 Total Pure urban 90 140 362 661 537 908 439 958 858 1424 6377 Mainly Urban 80-99% urban agglom. 109 136 185 227 172 291 187 212 252 173 1944 Mainly Urban 50-80% urban agglom. 193 209 407 369 256 352 175 255 246 160 2622 Urban with rural area Suburban (periferical urban) 991 569 576 229 83 29 5 1586 1256 1837 1340 831 860 388 375 310 212 8995 1 2483 Table 10 : Number of urban communes classified as urban or semi-urban. The geographical distribution of communes classified as “urban with a significant rural area” is quite uneven (Figure 7). In several countries they cover a large proportion of the territory. A modification in the threshold of the nucleus size might be recommended. Mapping rural/urban areas from population density grids 13 Communes with an urban nucleus and significant rural areas Nucleus size (*1000 inhab.) 5-10 10-20 20-50 50-100 100-200 200-500 500-1000 >1000 Figure 7: Communes with an urban nucleus and a significant rural area. The classification as “semi-urban” of communes that have part of the communal territory inside a small agglomeration needs to be reviewed. In most cases it corresponds to an artifact generated by the scarce spatial accuracy of the Landscan database used for the delineation of agglomerations and the excessive buffer applicated around the nucleus due to the coarse resolution of the population grid. Figure 7 shows an example of these effects: Laureana di Borrello (Calabria) is classified as a “commune with a small urban nucleus and a large rural area” in the category “between 5000 and 10000 inhabitants”. This may be debatable because the threshold of 5000 inhabitants may be too low, but this is consistent with the definition. 4 communes around are classified as “suburban” because of the coarse delimitation of the urban nucleus, with a too wide buffer, that appears to touch the 4 communes. Mapping rural/urban areas from population density grids 14 Rural communes classified as semi-urban Serrata Candinoni Laureana di Borrello Ferloleto Galatro 3 0 3 6 Kilometers Figure 7: Example of rural communes classified as suburban due to the coarse identification of urban nuclei. Peri-urban rural communes. In this draft classification, we consider rural all communes that do not intersect with the urban agglomerations identified with the criteria specified above. Some of these communes are close to urban nuclei and have some influence from them, for example part of the population may be working in the city rather than in the commune of residence. For a specific rural commune (that has not been classified as urban), we may want to know if it is near an urban agglomeration, and if this agglomeration is large or small. In order to classify communes into peri-urban and remote, we have to quantify the influence of an urban agglomeration. It is also useful to give a criterion to decide which is the most influent agglomeration to a given commune; the criterion should not be only based on distances, but take into account as well the size of the agglomeration. For example if a certain commune is 10 km far from an agglomeration of 10,000 inhabitants and 15 km far from an agglomeration of 1,000,000, the large agglomeration will have a stronger influence on that commune. Mapping rural/urban areas from population density grids 15 Defining the most influent agglomeration to a commune. A possible way to tackle the issue is defining an “area of influence” of an urban agglomeration through a buffer of a width that depends on the size of the commune; for example we could define a buffer of 5 km around communes of 5,000 to 10,000 inhabitants, and of 50 km around communes of more than 5,000,000 inhabitants. We have chosen an alternative approach based on a “gravitational index” To simplify the problem, both communes and urban agglomerations are represented by their barycentres. The influence of an agglomeration on a commune has been quantified through a “gravitational attraction” indicator: pop (a ) pop (a ) or G (c, a ) = 2 (1) G (c, a ) = 2 d (c, a ) d (c, a ) where c is the commune and a the urban agglomeration. The population of the commune c is not considered, because comparisons are made for each commune separately. For a commune c, the most influent agglomeration a is the one with the highest “gravitational attraction”: A(c ) = a ⇔ G (c, a ) > G (c, a′) ∀ a′ ≠ a Rural communes can be classified according to the intensity of this gravitational attraction indicator or according to the size of the most influent agglomeration. Communes are classified by levels of accessibility with this gravitational attraction index, from peri-urban to remote areas. A provisional classification scheme. With the draft scheme presented here, we would have three main classes of communes: urban, mixed and rural. Each class can be subclassified through several criteria. Figure 7 and Table 11 illustrate a classification in which rural communes are sub-classified according to the gravitational indicator given in the previous section. This scheme still needs a lot of improvements, but may be useful as a basis for discussion. Urban Mixed Rural Purely urban Mainly urban Suburban Urban with rural areas Close peri-urban Medium peri-urban Far peri-urban Medium remote Remote Severely remote Unclassified Mapping rural/urban areas from population density grids Number communes 6377 4566 8995 2483 4871 8721 15458 27461 17839 11157 767 Total Pop. (Landscan) in 1000 inh. 70012 123243 52699 88782 8176 13954 22214 36230 21041 13952 765 Total area Km2 19960 78235 340038 547725 78248 166910 348840 800096 660613 1067734 16 Table 11: A possible scheme of classification. Figure 8: A possible classification of communes Discussion Giving an absolutely objective criterion to classify a geographical area into urban and rural areas is probably impossible. Any method requires a choice of thresholds, that is subjective to a certain extent. A good method should be Mapping rural/urban areas from population density grids 17 flexible, so that a potential user can easily tune thresholds for a better adaptation to specific needs. We have considered the commune as the unit for classification The method we have used to classify the approximately 108,000 communes is strongly based on the previous identification of urban agglomerations that do not take into account communal boundaries. Many issues have not been addressed, for example how to deal with major tourist areas: How to consider a coastal commune that may have 2,000 inhabitants in the winter and 30,000 in August?. We use the terms “urban nucleus” and “urban agglomeration” in a rather unspecific and exchangeable way. It might be meaningful to classify into relatively small nuclei, often with an approximately round shape and complex agglomerations. A shape indicator, such as perimeter/sqrt(area), might be useful for this purpose. The results presented here are conceived as a basis for discussion. A number of arbitrary choices have been made, that need to be discussed, and some inaccuracies certainly appear, partly because of the population data grid used and partly because of the processing. If the approach is considered to be a valid basis, significant improvements are needed before submitting the results to validation by Member States. Some improvements can be made at short term, and some need a long-term work. At short term: • Some communes have not been classified. Some of them because the population density grid used did not cover the whole EU25: Cyprus, Malta, Atlantic islands, and Caribbean territories were not included. Others have not been classified because of recoverable problems in the GIS processing. • Better tuned criterion to measure remoteness. Figure 7 suggests that the importance of major agglomerations is too strong. Formula (1) might need an adaptation. • The class we have called “suburban communes” needs further reflection. The variety of situations included in it is too high. Many communes are classified in this group just because they are not too far from a medium size town. • Changing the size threshold for urban agglomerations; for example Vanhove (1999) suggests 30,000 inhabitants, and the Buckwell report (European Commission, 1997) considers a threshold of 50,000 inhabitants. At long term: Mapping rural/urban areas from population density grids 18 • • Visual inspection of the Landscan population grid in areas well known to the authors suggests that Landscan generally underestimates the permanent population of this type of rural communes. Using a better population density grid would lead to more consistent results, but this requires conditions that may be difficult to meet in the brief term, mainly availability of the 2001 census data by commune. CLC2000 would also be very useful for disaggregation. A useful proxy to estimate disaggregation coefficients would be a night time light emission map from satellite images. Distances to quantify remoteness have been measured in straight line without taking into account road networks, topography, etc. This should be improved. REFERENCES Bettio M., Delincé J., Bruyas P., Croi W., Eiden G., 2002, Area frame surveys: aim, principals and operational surveys. Building Agri-environmental indicators, focussing on the European Area frame Survey LUCAS. EC report EUR 20521, pp. 12-27. http://agrienv.jrc.it/publications/ECpubs/agri-ind/ CEC-EEA, 1993, CORINE Land Cover; technical guide, Report EUR 12585EN. Office for Publications of the European Communities. Luxembourg,. 144 pp. www.ec-gis.org DG Agri, 2004, GIS analysis of “rural” areas –EU25 (communal level), intermediate report. Dobson J.E., Bright E.A., Coleman P., Durfee R., Worley B, 2000, Landscan: a global population databasefor estimating population at risk. 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