8000

• UCREFRP Catalog Number: 8000
• Author: Werth, L. F., et al.
• Title: Developing a Geographic Information System for Resources Monitoring on the Colorado River in the Grand Canyon.
• USFW Year: 1993.
• USFW - Doc Type: R-93-20,
• Copyright Material: NO

The GCES/GIS data base is a multi-level structure which includes data that range from
general to specific. Three tiers of accuracy are available within the multi-level structure.
Each tier of accuracy is determined by either the scale of the base map or methodology used
in the development of the original data set. The first tier consists of data referenced to a
1:24,000 quad sheet and at best it will obtain the National Map Accuracy Standard of 40 feet
in the horizontal and half a contour interval in the vertical. The second tier of data are photo
referenced and transferred to the 1:2,400 orthophoto grid base map developed for this project.
The methodology used to create the 1:2,400 level can produce a digital product with a
horizontal accuracy of 2.0 meters and a vertical accuracy of 1.0 meter. This second tier of
data is being used in the development of the digital products for the thirteen monitoring
sites. The third tier of data is survey referenced data. This data collection methodology can
produce digital data with sub-centimeter accuracy. The GCES office has surveyors on staff
to assist contributing scientists in developing new data sets or georeferencing historic data
with local control (fig. 8). It is important to note that the accuracies mentioned here are the
best case scenarios and that the accuracy of contributed data depends on the methodology
used in the collection of the original data set.
Data Base Development
Initiation of a pilot study was determined necessary to develop a methodology prior to
committing the resources to all thirteen monitoring and two special study sites. The
Nankoweap Canyon area, site No. 4, was selected for the pilot because considerable historic
information was available, it was a critical area, and it would provide a good test of the
integration needs. The Nankoweap site begins at river mile 51 (according to the USGS) from
Lee's Ferry and ends at river mile 56.
Historic data, defined here as "data developed prior to the 1990 GCES/GIS base map," were
tested in the pilot study to see if they could be integrated into a valid and usable data set for
the GCES/GIS data base.
The black and white aerial photographs from 1984, with pin pricks representing the USGS
1923 river mileage, were used to test historic photo referenced data. A Saltzman Map
transfer machine allowed scaling and transfer of the 1923 mileage data to the 1990 base map
product using common monuments between the 1984 photography and the 1990 orthophoto
map. The base map was registered to a digitizing tablet and the 1923 river mileage data
were hand digitized into the GCES/GIS data base. This 1923 USGS river mileage coverage
was then used to integrate hard copy mapped data to the GCES/GIS data base.
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