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o (-j N ..... l:,'~ ....:I 1969. Census measures have been used in studies (8, 9) cited' widely to rebut the NALS data. The NALS estimate clearly suggests a more precipitolls land use con- version trend than does the census. At the root of the difference, however, are quirks in measurement methods. The data reconsidered The census land use statistics lire drawn from questionnaires iddressed to occupiers of land. Respondents base their estimates of acreages on totals specified in deeds or leases, which. in turn, are based upon measurements made at the time of the original rectangular surveys. The')c were made in Illinois between 18 t3 and 1849 (21) and wcre en.,hriried in plat books. In most areas, land de'icriptions continue to be based on plat book measurements. The plat books no doubt contain numerous er- rors, some of them sedous, but they do not appear to be systematlcally biased. Census totals based on farmers' replies need not be in serious error if the statistics are collected completely and without duplication. Review of the censlis data reveals over- counts of landRin-farms in many counties. Overcollnts were especially pronounced in 1978. but they also occurred in H!74 and 1969. The<;;e overcounts are partly hidden because the census also overstates county totals, of which land-in-farms is a major fraction, Comparisons with IIllnois State Geologi- cal Survey data show that census county totals in thl~ state tend ~o be overstated by about 1.1 pcrcent (5).. When the county totals are corrected downward (by Ii LOll ~ .9891). the presence and the size of over- counts of landRin~farl)1s become more clear. In some counties, such as Cham- paign County and surro~nding counties on Illinois' Black Prairie, Qvercounts are ob- vious, The census totals of land-in-farms are so large that entirely too little land is left (within the county total) to aCCclmt fcr urban land and other known nonfarm land use areas (4. 6). The problem is diffcreflt in counties with substantial forest acreage. In these coun. ties, we checked the cenSus figures against land use data collected Independently for the 1967 tJlinois Soil and. Water Conserva. tion Needs Inventory (ICNI) (3) and by the tJlinois Division of Forestry (lDF) (10). When checked against afrphoto surveys of several counties, the latter data were found to be consistent. The lCNl and lDF data were used to compute la.nd-in-farm totals fcr 1969 and 1978. However, because of uncertainties about the extent of recent ur- banization, we held the urban arca total at the 1967 level reported in the lCNl. Our 360 Journal of Soil and Water Conservation findings, therefore, should overestimate the land actually remaining in farms. Nevertheless, we found that Illinois farmland was overcounted by about 600.000 acres (2 percent) in the 1969 Cen- sus of Agricultnre and by about 1.5 million acres in the 1978 census (5 percent), Over- counts were found in about one-third of all Illinois counties in 1969 and about two- thirds of all lIIinois connties in 1978. Thus, even holding urban acreages constant at 1967 levels, the Census of Agriculture data conceal 900,000 acres of farmland lost be- twe",n 1969 and 1978. Additionallcsses at. tributable to urban acreage expansion could easily bring the total to 1 million acres or morc-a figure compatible with the NALS estimate for lIIinois rural land losses between 1967 and 1977. The census overcount can possibly be at- tributed to the change in enumeration methcds that occurred after the 1964 ccn. sus, That and earlier censuses were based on questionnaires completed by personal interviews. In 1969, 1974, and 1978, mail surveys were used, One hypothesis is that acreage duplications due to co-ownership or joint tenancy were weeded out more thoroughly in the personal interviews than in the impersonal mail surveys. Another indication of increasing ovef- counts in census farmland totals is that, be- tween 1964 and 1978, farmland acreages in many Illinois counties increased to un- reasonable levels as percentages of all county land. At the extreme, more than 100 percent of Menard, Douglas. Piatt, and McLean counties in central Illinois were farmed in 1978, according to the cen- sus. The proportions of land reported as farmed in the 1964 census in these counties wcre 92.3, 93.3, 96.7, and 93.5 percent, re<;pectively. One possible explanation for the excessive percentages in 1978 is that the land occupants in these counties came to control acreage in neighboring countie<;. However, increase., in farmland to within a few percentage points of all land charac- terizes census data for much of central Il- linois' Black Prairie, one of America's rich- est agricultural regions. These data conceal the areal expansions of cities and towns within the region after 1964, growth that is quite apparent to local residents. Chicago area trends The most dramatic examples of recent land conversion have occurred on urban fringes. A close look at trends in the six- county Chicago area helps reveal forces underlying this phenomenon. According to land use inventories by the Northeastern Illinois Planning Commis- sion (14. 15), 688,700 acres werc used for urban purposes in the mid-l nao:;; total land area was 2,3 million acres. By 197:-), an additional 312,700 acres had been con- verted to urban llSes, leaving 1.3 millioll acres as rural area. The Northeastern Illi- nois Planning Commission report has II similar magnitude specified in crops, ha~-, and grass in 1975 (15, Table LU.4). Of tile land de\'eloped into urban lIses, about 180,000 acres were developed bd\',/c(,rl HJ70 and 1975. Farmland conversion to urban uses oc- curred during a time of decline in the Chi- cago area's population growth rate. Dur- ing the 1970s, net outmigration from Chi. cago offset suburban county growth rates ranging from 10 to 30 percent, resulting in a net population increase of less than 2 per- cent. In contrast, during the lU60s the area grew at a rate of more than 13 percent. with suburban county grov.,th rates ap- proaching 60 perccnt (18. 20). The recent growth rate re..,>ults from a combination of depopulation of the ccntrai city and slackening population growth ill the suburbs. However, lower population growth has not been accompanied by a r~'- duced demand for land for urhan pmpn~cs or a slackening of farmland com.ersinn. Classic residential decentralization is lal.:' ing place. From 1964 to HJ75. both tho total urbanized acreage and the resicll'llti,d acreage rose more than 45 percent. whik the population increased about 5 p<"(Cl'flt_ More than 65 percent of the additional m- ban acre,> were located in the five frjllJ~1' counties. Factors other than population preSSIl{l' are driving land conversion to urban Il:,('.~ in the Chicago area. Six possible for('(\~ have been identified as shaping the IIfban structure (16): automobile transportation and the method of financing highwa~.~: house financing; tax policies affecting capi- tal gains, accelerated depreciation, and de- ductibility of property taxes ancl mortgage interest; tax exemptions for munidp<l! bonds; public utility pricing policies; and local government structure,>, Also, if spa('t' is a normal good, boosts in real income in- crease the demand for space and urban land. During the 1960s and the first hall of the 1970s real per-capita income in the Chicago area grew at the compound an- nual rate of 1.82 percent, reaching $7.Wi in 1975 (current dollars) (2). Decentralization has accompanied thi., greater demand for space by residents ill the Chicago area, Gross residential arc;! per 1,000 people increased from 39 acres ill 1964 to more than 54 acres in 1975. In the suburban counties, the average was 120.:J residential acres per 1,000 people in HJ7:J This ratio was 91. 7 in 1964, CommerciaL' .