Visualization of ICESat Shots in Google Earth (VISGE) Overview
The Visualization of ICESat Shots in Google Earth (VISGE) tool is an experiment which NSIDC hopes will assist users in determining whether there are usable GLAS data over a particular area of interest. Useable data are defined as data with minimal cloud cover and with an acceptable amount of saturation and/or gain. This page provides an overview of how and why this tool was created. To download the tool, please see the NSIDC Virtual Globes: Technical Experiments Web page.
With over 1.5 billion laser shots now in the NSIDC archive, tools to assist users with the data are needed. To address this need, NSIDC has created the VISGE tool which consists of a KML file that allows users to browse select GLAS holdings using Google Earth. This should decrease the time spent searching for, ordering, and inspecting the data. A user will be able to view the approximate area covered by each laser shot’s footprint, color-coded based on gain and a saturation flag, on top of Google Earth’s imagery. Clicking on the coincident and similarly colored pushpin displays the associated waveform in a popup window, helping the user determine whether the shot is of acceptable quality before ordering any data since analysis of the return pulse, or waveform, is the basis of all information derived from GLAS.
For this experiment, NSIDC chose an area around Anchorage, Alaska USA for its diversity of snow, ice, water, forest, and urban surface types. To make the data volume more manageable, the area of interest has been limited to a 5x5 degree square. Each laser campaign, Laser 1A to Laser 3J, contains between 10 to 18 orbit tracks in this square.
A footprint is the area illuminated by the transmitted laser pulse. For altimetry data, the GLAS laser transmits short pulses (four nanoseconds) of infrared light (1064 nm) 40 times per second, yielding over 110 million shots per 33-day laser campaign period. The spatial resolution of the disk illuminated on the Earth's surface by the laser is approximately 70 m in diameter, and shots are spaced 170 m apart.
The GLAS return pulses are digitized onboard the spacecraft and are temporally bounded by a priori information from a 1 km resolution Digital Elevation Model. In the ground system, two different algorithms are used to evaluate the waveform, depending on the surface type. For relatively smooth surfaces, such as ice sheets, sea ice, and oceans, a single Gaussian peak is fit to the returned waveform. Over land, where there may be returns from multiple surfaces having different elevations within the footprint, multiple Gaussian peaks (up to six) are characterized.
The waveform reveals characteristics about the surface (and intervening atmosphere) at the associated footprint. Much of what researchers are looking for (elevation, surface type, slope, etc.) can be gleaned from the waveform shape and intensity. Since the yearly campaigns repeat the same orbit, change detection is possible. To properly interpret and quality check the elevations produced by GLAS, inspection of the waveforms, as well as the quality indicators, is essential. The goal of the VISGE tool is to help with this inspection.
The figure below is a truth table that shows how the color-coding of the footprints and pushpins is determined. The parameter used to indicate saturation is the i_satElevCorr parameter. The parameter used to indicate gain is the received pulse gain value, i_gval_rcv.
|Figure 1. Truth table describing color-coding of push-pins. For the gain parameter, acceptable gain values are 13-49; unacceptable gain values are 50-255; and the values 0-12 are not used. For the saturation parameter, a value of 0 indicates no saturation, and any other value indicates saturation.|
In order to aid in the future development of this tool, please share your feedback on the usability of this interface by taking the VISGE Usability and Feature Request Survey.