IfSAR: the Southeast ‘big-picture’

Posted 2016: IfSAR stands for interferometric synthetic aperture radar. While many scientists, naturalists and outdoorspeople in Juneau have now heard of LiDAR—because of the extraordinarily detailed topographic detail and forest structure maps it’s given us in the CBJ—fewer know about a slightly lower resolution dataset that was acquired throughout the state between 2010 and 2014, and has finally been processed and delivered. It can be downloaded from the GINA website.

IfSAR coverage is nearly complete for Southeast Alaska.

Because I wanted the entire collection for Southeast Alaska, I’ve bypassed the download process and copied a drive prepared by Dustin Wittwer, USFS, cataloging all of the IfSAR output south of Yakutat.

The Alaskan IfSAR project consists of 3 basic products, from which many others can be spun off in GIS: bare-earth DEM (DTM, or digital terrain model), Surface DEM (DSM or digital surface model), and an orthorectified radar intensity image (ORI). The elevation models have a 5-m post spacing, 22-ft contour accuracy, 3-m vertical accuracy and 12.2-m horizontal accuracy.

As a landforms enthusiast, I’ve been enthralled by Juneau’s LiDAR bare-earth display for the past 2 years. In fact, it’s spoiled me to the degree that consulting or teaching gigs in other parts of Southeast have begun to feel cartographically impaired. So the sudden delivery of the IfSAR DEMs for almost the entire archipelago is like Christmas 2 months early! Thank you, GINA and Dustin, and Dave Gregovich at ADF&G for your pioneering analysis of this dataset.

Update, 2018: More and more communities are commissioning LiDAR, and for many cartographers  it seems as though this reduces incentive for diving into the coarser-resolution IfSAR coverage. But it will be many years (if ever?) before we have complete LiDAR for all of Lingít Aaní. Meanwhile, whenever I ask ‘big-picture’ landscape questions, the IfSAR cover is my go-to resource.

For example, to extend subshed mapping from our LiDAR-surveyed CBJ road system back into the icefield, I turned on the IfSAR model, noting that this gave me much more accurate boundaries than either HUCs (USGS) or VCUs (USFS).

Glacier boundaries are another great application for IfSAR. Existing layers are derived from coarse-resolution satellite imagery that includes a great deal of non-glacial snow and ice. On receipt of a complete IfSAR layer for Southeast Alaska, used topographical inflection and texture, rather than imagery, tracing for 3 days, and applying names from a point-file of Alaska glaciers. IfSAR-generated contours also allowed me to map the ‘headwater’ divides between some glaciers. This new glaciers layer is now loaded to ArcGIS Online.

Landforms traced onto IfSAR bare-earth, at Ltu.áa, bay inside the nostril (Lituya Bay)

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Southeast glaciers from IfSAR, ArcGIS Online

On receipt of the new DSM (digital surface model) from IfSAR missions at 5-meter pixel resolution, it became possible to…

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