ICESATPROCESSOR, AN INTERFACE FOR ICESAT DATA EXTRACTION: APPLICATION GUIDE

Transcription

1 ICESATPROCESSOR, AN INTERFACE FOR ICESAT DATA EXTRACTION: APPLICATION GUIDE September, 2017 Otá vio Augusto Pássáiá Rodrigo C. D. Páivá

2 1 INTRODUCTION This manual aims to present an application guide for the ICESatProcessor, an interface for ICESat (Ice, Cloud, Land and Elevation Satellite) data extraction focused on hydrological applications. ICESat is an altimeter satellite developed by NASA. ICESatProcessor is a set of tools developed through Matlab subroutines for extraction and visualization of ICESat altimetry data for water bodies. This application guide presents a case study at Guaíba River region, located in the state of Rio Grande do Sul, Brazil. In the following article you can search further information: Passaia, O. A.; Paiva, R. C. D. ICESat Data Processing for Hydrological Aplications. Revista Brasileira de Recursos Hídricos. Submitted The ICESatProcessor user can define: a) the study period as day/month/year (for example, 08/11/2003 to 08/11/2004); b) the study site (for example, Guaíba River), in two ways: i) latitude and longitude limits; ii) a polygon within the limits of its region, previously developed in a Geographic Information System (GIS) program; b) the use or not of SRTM DEM as a criterion for the removal of outlieds. There are some indexes (flags) that contain information about the observation, such as the elevation-use flag (i_elvuseflg), which is used to identify appropriate observations. The saturation index was also used to remove or correct saturated observations. Observations with index three or greater were removed. An index of zero or one is suitable for use without correction because there is no saturation or there is an unimportant level of saturation, respectively. Those with index 2 had saturation correction applied to them. After that, it is necessary to change the reference system. Therefore, the data are converted from the Ellipsoid T/P to the World Geodetic System ellipsoid of 1984 (WGS84), and then to the Vertical Earth Gravitational Model datum of 1996 (EGM96). Geoid EGM96 was chosen for this study because of its common use, particularly in hydrodynamic modeling of large river plains where Shuttle Radar

3 Topography Mission (SRTM) data, which is also referenced to WGS84 EGM96, are commonly topographic data sources (HALL, 2012, O'LOUGHLIN, 2016). However, with only these spurious data removal criteria, there are still many unusable data (outliers). In an attempt to remove them, a new criteria was used: compare ICESat measurements with SRTM. The study of Carabajal and Harding (2005) showed that the difference between the observations of the two missions for the worst case (a lot of vegetation covering the ground) is, on average, m (average and standard deviation). Therefore, in the software developed in this study, if the difference is greater than 50 m, the observation is removed. The following flowchart illustrates all steps performed in the program. Figure 1 Software operation flowchart ICESatProcessor can also compute water elevation time series for Virtual Stations. According to Silva (2010), "Each intersection of a trace of the altimetric satellite with reflective surface of the water plane consists of a virtual station, being possible to obtain a time series of the height of the water level". For each intersection, the ICESatProcessor will calculate the mean, median, and standard deviation of the series, and identify the outliers in two ways (in addition from those already mentioned: a) points that are likely to belong to the

4 population less than or equal to 5% (considering a normal distribution); and b) observations outside the quartile limit plus 1.5 times the interquartile range. 2 TUTORIAL The procedures described in the last three paragraphs are performed internally by the software. Hereafter it is show each step to be performed for its use. The steps are bold and underlined. It is necessary to download data from the National Snow and Ice Data Center (NSIDC) website, available at < (ZWALLY, 2014). In this raw data packet, there is information about latitude, longitude, elevation, about wave peaks, and various flags that can be used for an initial screening. The raw data has been attached for a few days, so you can test the software while you do not have all the data yet. The ICESatProcessor must be installed. Before that, you must have the correct version of Matlab used to compile. Double-click MCRInstaller.exe and install MatlabRunCompiler. When finished, click on ICESatProcessor, and its interface (figure 2) should appear on the screen. It was all written in English, to enable worldwide use. Figure 2 Software Interface

5 Usually this step is not required but, if prompted, click on the ww15mgh.grd file. During the more time-consuming steps, while an operation is performed, there is a "Please Wait" window. When the operation is finished, a window called "Sucess" will appear saying "Operation Completed". Well, click ICESat Data Folder and select the directory where the raw data is downloaded (in this example, NASA Raw Data). Afterwards, select the output folder in Output Folder. The spatial boundaries can be entered manually, always from left to right and from bottom to top. For example, Latitude: and Longitude Or you can simply click Select Shapefile and your polygon boundaries will also be the latitude and longitude boundaries. There is a sample polygon called Guaiba.shp. In the button SRTM DEM Selection, you decide whether to use the SRTM MDE as a criterion for spurious data (outliers) removal, or not. If you wish, check the box on the left, and select the.asc file that contains the grid corresponding to your study region (a grid of this example was placed, SRTM_26_19). It is possible to download boxes for the whole world at Now you will select the period for which you want to run the application. The default dates are the entire satellite period. You can select the ones you want, and then click Availability. This step is necessary because the satellite does not have data for every day of the year. If the chosen dates correspond to ICESat dates of passage, messages will appear stating that the dates have been found. Otherwise, change the days to those in which the satellite passed. For this example, chose 08/11/2003 a 08/11/2004. In the button Spatial Distribution the user can view satellite orbits over the region of interest. The data is not processed, only extracted for that area. The output is a text file with the longitude; latitude; elevation; year; month and day. At this point, the ICESatProcessor interface should be as follows:

6 Figura 3 Software Interface at the example At Extract Data the software handles the data as described above. The outputs are three text files: READ-ME; Results; and ICESat and SRTM. The READ-ME file shows the days in which the satellite passed over the region of interest, how many observations ("points") are in the area, how many were excluded based on the elevation flag and how many were removed based on saturation flag. The Results file has the same format as the Spatial Distribution. The output file of the ICESat and SRTM type has: longitude; latitude; elevation of ICESat; SRTM elevation; elevation used; year; month and day. The option Virtual Stations is similar to Extract Data, besides handling the data and generating the file READ-ME, There are two more text documents, called ResultsShapefile and Virtual Stations. The first is similar to the Results file, however only with the observations inside the polygon, and with an additional column (ID) indicating the number of the polygon in which the station was made. In the second, the columns show, from left to right: elevations average; median; standard deviation; longitude; latitude; year; month; day; number of points used; index of the polygon where the station was made; number of points removed by the criteria in the last paragraph of the introduction. When using ICESatProcessor, quote the reference as follows:

7 Passaia, O. A.; Paiva, R. C. D. ICESat Data Processing for Hydrological Aplications. Revista Brasileira de Recursos Hídricos. Submitted If you have any questions, suggestions, difficulties or distress, you can write to the software developer via REFERENCES Hall, A. C.; Schumann, G.; Bamber, P. D.; Bates, P. A.; Trigg, M. A. Geodetic corrections to Amazon River water level gauges using ICESat altimetry. Water Resources Research. Florida, v. 48, n. 10, p. 1-6, Jun O'Loughlin, F.; Neal, J.; Yamazaki, D., Bates, P. ICESat derived inland water surface spot heights. Water Resources Research, Florida, v. 52, n. 4, p , Abr Silva, J. S. Altimetria Espacial aplicada aos estudos de processos hídricos em zonas úmidas da bacia Amazônica p. Tese Doutorado em Engenharia Civil Curso de Pós-Graduação COPPE/UFRJ - Université de Toulouse, 2010 Zwally, H. J. GLAS/ICESat L2 Global Land Surface Altimetry Data (HDF5), Version 34. GLA14. Boulder, Colorado USA. NASA National Snow and Ice Data Center Distributed Active Archive Center. doi: Accessed in 26/05/2016, 2014.