SNODAS SWE Data on PSU CSAR Server

Please use the following web links and instructions to view or access the data.

SNODAS Daily SWE Web Map

To view daily SWE, go to http://atlas.geog.pdx.edu/SNODAS/ using IE web browser (make sure your browser doesn't block any javascript). You can use the pull-down list below the map to select a SWE image file and click on "Update Map" to view the image. The name of the image file has a format of SWE_YYYYMMDDHH.img. YYYYMMDD indicates the date of the modeled SWE. The hour is always 05 (i.e., 5 am) for all the image files. Daily SWE data will be updated and available on the CSAR server by 7 am daily.

Daily SWE FTP Server (Sep 30, 2009 - present)

To download the SWE .img file, go to ftp://atlas.geog.pdx.edu/SNODAS_SWE/. You can open the link in a web browser or type in the path in the address bar of Windows (file) Explorer. I find it easier to copy the files from Windows Explorer. The SWE .img files are organized in subfolders named by year and month (for example, 201306 is for June 2013). Each SWE .img raster data has 5 associated files that have the same file name but different file extensions (.img, .img.aux.xml, .img.vat.dbf, .img.xml, and .rrd). These files are compressed into a zip file. Please copy the zip file to the local drive and decompress it. The .img file is in "USA Contiguous Albers Equal Area Conic USGS" projection. The spatial resolution is 923m by 923m. The ftp server has data from September 30, 2003 to present.

Time-aware SWE Image Service

The SNODAS SWE dataset is also available through the CSAR server as a time-aware image service. For a description of the web image service, see the metadata page of the service. Users can display the historic SWE layers in ArcMap as a time-lapse animation or select a layer from a specific date of the image service and use it in analysis. To use the service, you must connect to the PSU CSAR atlas server from ArcMap (see connection instructions here). Then, add the SNODAS_SWE_TimeAware service to ArcMap (Figure 1). The file size of the source images of the image service is large. It will take a few minutes for ArcMap to load the image.

Figure 1.

To view a specific date of a time-aware image (service), right-click on the layer and open the layer properties dialog window. From the layer properties dialog, select the "definition query" tab and click on the "Query Builder" button to open the query builder (Figure 2). You can use the buttons in the dialog window to construct a SQL statement. When finished, click OK to dismiss the Query Builder window, then click OK to close the layer properties window and display the SWE image from the specified date in ArcMap (Figure 3). You can use the image service layer to perform geoprocessing tasks in ArcMap.

Figure 2.

Figure 3.

Enabling time in ArcMap rather than picking a single date to display via a definition query, one can enable time in ArcMap and scroll through all available images chronologically. To do so, first enable time on the image service layer: in the layer properties, click on the Time tab. Check the box to enable time on the layer. Ensure that the time field is Time, the field format is <Date/Time> . The time set interval must be set to 1.0 Days, as the data has daily resolution (Figure 4). 

Figure 4.

With a time-enabled layer, time needs to be enabled in the map document via the Time Slider. The Time Slider is the second-from-the-right icon in the Tools toolbar (Figure 5). Click it and time should be enabled in the map. If not, the icon in the upper left corner of the Time Slider window will enable time in the map. With time enable in the map, the user should be able to drag the time slider to move chronologically through the images in the image service (Figure 6).

Figure 5.

Figure 6.