The following is an excerpt from a paper submitted by RECON Enviromental,
Inc. Titled "Land Cover Data Assessment in Pima County"


PAG208

In the 1970s, Pima Association of Governments commissioned a
set of maps for the county (excluding metropolitan Tucson)
delineating vegetation, landform, slope, geology, and soils.
Maps were compiled by the Applied Remote Sensing Program at
the Office of Arid Lands Studies (OALS), University of Arizona,
and completed in 1977. Resource boundaries were delineated
using 1972 and 1973 aerial stereophotographs and registered to
USGS 15-minute quadrangles. Vegetation and soils were classified
based on 1,000 field records plus ancillary data (STAT 1999).
Field records were logged on data cards which are associated
with point data in a GIS coverage. All polygon mapping remains
in hard copy format. Vegetation classification is based on
dominant/subdominant species and gross physiognomy, which
together with data cards could be cross-walked to BLP biome or
series for many types.  PAG land cover mapping was compared
with GAP and the Cienega Creek Natural Preserve study to
determine whether PAG maps could be useful in improving GAP
mapping (June 1999). This comparison showed that PAG mapping
is not reliable in riparian areas since these have changed
significantly since the 1970s. And upland areas are not
comparable since PAG vegetation classification is unique. PAG
data cards could be useful in improving vegetation
classification, but it would be unclear where to draw the
boundary for these classes, and PAG's relatively course scale
(20-acre minimum mapping unit) does not help resolve mapping
issues for small areas. Given the level of effort required to
digitize these hard copy maps, PAG data should be used only for
very specific investigations (i.e., searching for a particular
feature).