elevation

Glacier Stake Locations

Abstract: 

As part of the Long Term Ecological Research (LTER) project in the McMurdo Dry Valleys of Antarctica, a systematic sampling program has been undertaken to monitor mass balance and meltwater flow from the Taylor Valley glaciers. This data set contains global positioning system (GPS) measurements of stakes on glaciers. The measurements include both low-quality estimates (+/- 100m) from hand-held GPS units and high precision measurements (+/- 2 cm) using Trimble (4000 SSE) geodetic receivers. The purpose of these measurements was to obtain rough stake locations for use in relocating stakes in the future and precise stake locations for use in calculating stake displacement. The measurements are made relative to the WGS84 ellipsoid.

Additional Project roles: 

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Data set ID: 

2000

Short name: 

glstkloc

Methods: 

 Handheld measurements were acquired using either a NAVPRO 5000 or GARMIN 45 GPS unit. Latitude and longitude accuracy are about +/- 100m. The handheld units did not provide adequately accurate elevations, so elevation was acquired from the 1971 map of the valley. There was no special procedure in acquiring the data or exact location relative to the stake.
 
The precise GPS measurements were acquired with help of UNAVCO. (See photos and further  descriptions below). Measurements were made using a Trimble 4000 SSE receiver. A base station was set up prior to measurements in the field. The  baseline between the measurements and basestation did not exceed 15 km. The measurements were collected as rapid static surveys.
 
For the precise measurements, the GPS antenna was mounted on top of a 2 meter  tall rangepole. The rangepole was placed next to the each ablation stake on the down-glacier side of the stake, and the tip of the rangepole was placed on the  board used for mass balance measurements at that stake to insure that the measured elevation was relative to the same surface used in the mass balance  measurements. This system resulted in the rangepole sitting 2-3 cm away from the  down-glacier side of the ablation stake. Repeatibility of placement was  ascertained to be about +/- 2 cm based on repeat measurements. The rangepole was  always vertical, as indicated by a level-bubble.
 
At each stake, data were collected for 10 - 20 minutes, sufficient to keep the horizontal precision of the GPS results within 2 cm. Accuracy of the resulting measurements are estimated to be 5 cm in the horizontal and vertical,  after accounting for set-up error.
 
During the 1995/96 season, the position of all the ablation stakes on four different glaciers were measured using rapid-static GPS. A re-measurement in the 1996/97 season provided displacements and velocities. 
 

Data sources: 

GLSTKLOC

Instrumentation: 

NAVPRO 5000

GARMIN 45 GPS unit.

Trimble 4000 SSE receiver

Maintenance: 

In 2015, Inigo San Gil moved these data to the Drupal Ecological Information Management System, thus modernizing a bit the way data is managed and distributed.

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Data for the original stake locations was submitted by Andrew G. Fountain to the data manager in August, 1997. The column showing "file name" identifies the original file containing that record. These are ascii text files and can be found in the McMurdo LTER data manager's home directory on INSTAAR's Unix system.

Once submitted to INSTAAR, the data manager used Microsoft Excel and Access software to produce files that were in more of a relational mode. The resulting files are represented in the "glstkloc.dat" file (ascii text, comma delimited format), and "glstkloc.txt" file (MS-DOS text format).

Initially, there was no date associated with each record in the data file. However, due to constant movement of ice on glaciers, the stake locations required subsequent measurements during the 1996-97 field season. When this occurred, a field for date was added to this file. Dates from the initial GPS measurements were acquired from the raw data files submitted by Andrew Fountain. If uncertain about what date to use, confirmation was made with Karen Lewis.

Upon verifying altitude corrections for the gps locations with Bjorn Johns of UNAVCO (in March, 1998), Karen Lewis discovered that all of the ellipsoidal heights to sea level obtained by precise GPS measurements were too short by 55.24 meters. Denise Steigerwald therefore adjusted the elevations for these stakes by adding 55.24 meters.

In April, 2000, Denise Steigerwald added fields for dataset code and glstkid. Dataset code would allow the data to be linked to the metadata in a relational database. Glstkid is a code that ties stake records to points on a GIS base map being developed by Michael Prentice at the University of New Hampshire. In addition, Denise:

  • Removed records associated with "B" (replacement) stakes. Coordinates for these stakes match those provided for the original stakes. Data sets containing replacement stake records had their stake values divided into 'stake' and 'stk replcmnt' fields, thus allowing the 'stake' fields to be linked between the locations file and the data files.
  • Converted all latitudes and longitudes from degrees-minutes-seconds to degrees in decimal format. This would allow them to be stored in numeric format rather than as text.

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