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LF National Map Download Extent Size: Unable to download a large area of interest (Download extent rectangle turns red)

Background:

A file size limit has been placed on downloads to equitably use bandwidth.

Advice:

There are three restrictions that are placed on downloading data through the LANDFIRE National Viewer.

The first limit is within the map viewer itself.  Every single layer has a weight assigned to it.  This weight is loosely based upon the average megabytes per degree for that dataset.  When the user draws a download extent box on the map, the sum of the weights for all layers turned on in the download layer list is multiplied by size of the extent box, to get a rough estimate of the megabytes in the download request.  If that result is larger than a predetermined factor (currently 1,500 MB), the box color changes from green to red.  If the user tries to select a download area that is too large (red box), they will receive a warning that the request is too large to receive as an on-line download.  To avoid receiving this error message, reduce the number of layers selected in the download layer list, or draw a smaller extent for the download request.  

The second limit is the maximum file size in megabytes that can be downloaded as an on-line download, which is calculated in the Request Summary page for each download request.  The request summary will intersect the download request with the data to get a precise amount of coverage for each dataset contained with a download request extent.  It then multiplies the amount of coverage with the average megabytes per degree for a dataset, to get a fairly accurate estimate of the size of the download request.  If the download request is larger than a predetermined factor (currently 1,500 MB) then it will not allow the user to download that request.  To avoid encountering this limit, the request needs to be split into multiple download requests of smaller size, or with fewer datasets at a time.

The third limit is the size of the download pieces, which is also calculated in the request summary page.  The download method used by the Seamless Data Distribution System is a type of HTTP pull that directs users to temporary download files.  These download requests occur once, and if there is an error, the user needs to create a new download piece rather than just refresh the page.  This means that each download must start and finish in one request.  The maximum size of those pieces is restricted (currently 250 MB) to keep users from requesting lots of large pieces that they are unable to finish in a single request.  This size defaults to 100 MB and can be set using the Request Options page to be smaller or larger.  

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