Survey Control Operations for the Colorado River Ecosystem
The Survey Control Network is fundamental to the spatial positioning of all scientific data collected by GCMRC's scientific monitoring and research programs. Accurate spatial positioning of scientific data facilitates integration across resource areas by providing common geographic framework to store and analyze data. Many resource monitoring programs depend upon changes in the spatial distribution of resources as the basis of their monitoring strategy. Spatial analysis tools such as a Geographic Information System (GIS) depend upon accurate geo-referencing of data to provide meaningful analysis. Without geographic control, geo-referencing of resource data and subsequent spatial analysis is impractical.
SURVEY CONTROL
The control network is the positional infrastructure for all surveying, mapping and remote sensing operations in the Grand Canyon. The primary geographic reference system along the Colorado River Ecosystem (CRE) are the survey control points (right), which consist of well-defined and monumented points within the study area. Currently, only about a quarter of the CRE has adequate geographic control to meet the needs of near and long-term monitoring and research plans (river mile –15 to river mile 72).Survey control is required throughout the remainder of the CRE to fully implement the long-term monitoring and research activities.
Control point coordinates are established using GPS and conventional survey practices. The seven existing rim control points were GPS surveyed by GCMRC Survey for Federal Base Network Surveys, for inclusion into the national High Accuracy Reference Network (HARN). The goal was to establish GPS derived positions for rim control points surrounding the Grand Canyon for all future CRE surveys. The following rim control points have fixed coordinates from which the coordinates along the CRE are referenced; L404 at Page, EMIN at Eminence Break, DSVW near Desert View Watchtower, SIGN at Great Thumb Mesa, DAVN at Toroweep Overlook, T96 at Peach Springs, and AIRP at Meadview.
The coordinates for selected river corridor control points are solved through simultaneous GPS occupations with the established rim control points. Conventional survey techniques are used along the CRE where steep canyon walls obstruct satellite visibility for GPS observation.
The positional accuracy of rim control points is constrained to 2 centimeter vertical and 1 centimeter horizontal accuracy standards in order to achieve the 5 to 8 centimeter level at the river (determined as four times more accurate than scientific data requirements of some of the GCMRC projects). Positional accuracy from the rim is transferred to the river corridor control points, and is subsequently, transferred to all mapping along the river corridor through conventional surveying techniques.
GPS Surveying
GPS satellite geodetic surveys use “relative positioning” where two or more receivers are collecting simultaneous dual frequency measurement data. The data is processed and distance vectors are solved by triangulation between data from each common satellite collected by the receiver operating on the known fixed position and the receiver operating at the unknown position (below). If the same four satellites are seen from each point, then the four variables can be solved to calculate the vector; dX, dY, dZ, dT. If more than four satellites are available, the solution becomes stronger. For (n) number of receivers collecting simultaneous data, there are (n-1) vectors that can be solved between points. The more vectors that can be solved between known and unknown points, the more accurate the survey network becomes. In Grand Canyon, where elevation masks often exceed 40 degrees, there are fewer satellites available on the canyon floor for GPS to solve for all four variables therefore conventional survey methods are used to compliment the GPS technique.
The GCMRC survey department tries to minimize all potential impacts to the Grand Canyon river environment. Whenever possible, existing control/survey monumentation is utilized. This includes bolts, X-cuts, nails, and survey monuments. However, in some cases it is necessary to set new points. These points are very carefully selected to be unnoticeable by the average person. When necessary, a small X is scribed on a rock. Extreme care is always taken to avoid any survey operations on or near environmentally sensitive or archaeological sites unless that site is specifically being mapped.
In order to meet GCMRC’s positioning needs, the existing control reference system must be continually enhanced to provide the high accuracy requirements needed to support all GCMRC scientific monitoring and research.