Surveying and Land Information Science

Vol. 62, No. 4 [December 2002]

 

The Effect of Broadleaf Canopies on Survey-grade Horizontal GPS/GLONASS Measurements

Thomas H. Meyer, John E. Bean, C. Roger Ferguson, and James M. Naismith

 

A study was conducted to empirically determine the degradation of survey-grade GPS horizontal position measurements due to the effects of broadleaf forest canopies. The measurements were taken using GPS/GLONASS-capable receivers measuring C/A and P-codes, and carrier phase. Fourteen survey markers were chosen in central Connecticut to serve as reference markers for the study. These markers had varying degrees of sky obstruction due to overhanging tree canopies. Sky obstruction was measured by photographing the sky with a 35mm reflex camera fitted with a hemispherical lens. The negative was scanned and the image mapped using an equal- area projection to remove the distortion caused by the lens. The resulting digital image was thresholded to produce a black-and-white image in which a count of the black pixels is a measure of sky-area obstruction. The locations of the markers were determined independently before the study. During the study, each marker was occupied for four 20-minute sessions over the period of one week in mid-July, 1999. The location of the study markers produced relatively long baselines, as compared with similar studies. We compared the accuracy of GPS-only vs. GPS&GLONASS as a function of sky obstruction.  Based on our results, GLONASS observations did not improve or degrade the accuracy of the position measurements.  There is a loss of 2mm of accuracy per percent of sky obstruction for both GPS only and GPS&GLONASS.

 

GPS Control Densification Project for Illinois Department of Transportation District 3

Shanlong Kuang, Coventine Fidis, and Floyd Thomas

 

This paper describes the concepts, instrumentation, software, and field as well as office procedures used to establish horizontal control for the Illinois State Department of Transportation (IDOT) District 3, using advanced Global Positioning System (GPS) technology. The IDOT District 3 is located in east central Illinois and covers 11 counties: Kendall, LaSalle, Putnam, Marshall, Woodford, McLean, Livingston, Grundy, Ford, Iroquois, and Kankakee (see Figure 1). Due to intensive district-wide development over the last two decades, the existing National Geodetic Survey (NGS) horizontal control system is not sufficient to meet the ongoing needs for project survey control. The Illinois Department of Transportation is, therefore, seeking to densify and upgrade the existing NGS horizontal control throughout the District to serve the district-wide needs for land acquisition, map production and design, Geographic Information Systems (GIS), and any other construction- or maintenance-related survey operation in the future. ASC American Surveying Consultants, P.C. has been selected by the IDOT District 3 to perform this task. The project area encompasses approximately 11,000 square miles.

 

Expected Positions of Standard and Closing Corners in Public Land Survey System Townships

David Ciampa and Wesley Parks

 

The distance between the expected positions of standard and closing corners in PLSS townships is a function of latitude and nominal distance of the corner from the meridian. A combination of low latitudes and closeness to the principal meridian can yield a minimal distance between standard and closing corners. Where corners are set by surveys meeting standards of PLSS Manuals of Instruction, actual positions of both standard and closing corners lie within error ellipses reflecting allowable positional error. Situations exist where work meeting Manual specifications could have resulted in a closing corner lying on the unexpected side of the corresponding standard corner.

 

An Analysis of Remote Sensing, GIS, and World Wide Web Utilization in Geoscience Education  in the U.K.

Serwan M. J. Baban

 

The study aims to examine the usage as well as the potential future use of Geographical Information Systems (GIS), remote sensing, and the World Wide Web (WWW) for teaching and learning the Geosciences in higher education institutions in the U.K. via information gathered through a questionnaire. The questionnaire was sent to all geography departments in the U.K.. Forty-one departments returned it, giving a response rate of 51.2 percent. All institutions were ranked based on their use of each technology; an overall usage ranking was calculated as well. Institutions achieving the highest scores for remote sensing were Southampton, Swansea and Edinburgh Universities. For GIS they were Leeds and Edinburgh Universities and for the WWW it was Southampton University which was also the highest overall ranking institution. A large proportion of the institutions surveyed were not fully aware of the variety and depth of information accessible via the WWW and of the potential of its use. The analysis revealed that there is a correlation between the individual use of these technologies within each institution and an overall expectation for increasing the usage of all technologies in the future. Traditional universities had the highest ranking for each technology as well as the highest overall usage of all three technologies.

Keywords: GIS, remote sensing, WWW, Geosciences, U.K.

 

In What Sense a Free Net Adjustment?

Willie Tan

This paper examines the philosophy and mechanics of free net adjustments with the aim of clarifying their strengths and weaknesses. The adjustment is applied to a simple leveling network.

 

Understanding “Protracted” Surveys

Dexter M. Brinker, PE-PLS

 

The most troublesome concept in the arena of Public Land surveys is the “protracted” survey. In order to appreciate how the system operates, we first need to consider one of the major problems confronting our country in 1785, shortly after the Revolutionary War. The new nation had acquired a vast area of “public domain” land, and a method was needed to quickly divide it into “farm size” units for orderly disposition. Some of these parcels of land (approximately 160, 80, or 40 acres each) were to be used in lieu of money to pay soldiers for their army service, since the government had not yet invented “deficit financing.”

Certainly, Thomas Jefferson and those who helped develop the Public Land (Rectangular) Survey System realize the enormity of this job. Rather than attempting to survey all the individual tracts, a two-stage system was devised which provided for a framework of surveyed and monumented nominal square mile units, called “sections,” augmented by rules of procedure under which these sections would later be subdivided into the smaller desired parcels. These tracts would be “created” by first platting them on paper, following a standard plan. Hence the term “protracted,” meaning to “draw out” (see Chapter III, Manual of Surveying Instructions, 1973, Bureau of Land Management).