Jan 07 2011
The ESRI Community Basemap program’s new ArcGIS 10 template for Large-Scale Topographic Mapping is somewhat streamlined and cleaner when compared to its ArcGIS 9.3.1. predecessor. But the template map document still has the ability to bring my workstation to its knees when doing stuff like printing. I find myself unable to print 11×17 or export a B-size PDF without having incomplete results. Mercifully, I can print and export to A-size so people are starting to get a taste of what is to come when we start building cache tiles. Everyone who has seen the base maps was quite pleased, and some were almost gushing over them.
Preparing input features to pour into the template geodatabase that are good enough to be worthy of 1:1200 scale mapping is every bit the challenge it might seem. Each feature class that is making its way into the template seems to require a unique bit of spatial analysis to get pulled together, and usually this involves combining our best available input data in ways that we haven’t before. This week’s challenge has been water polygon and water line feature classes. For utility, we’re choosing to split out the water polygon from its water line. For lakes and ponds, they will coincide, but for our tidal coast, we’re conveying extra information by setting the polygon to represent Mean Low Water, and the water line to represent Mean High Water. This helps to convey the widely varying slopes that Marin County has at the tidal shores, and provide some hint as to the width of the public access way below the high tide line.
These tidal shores have been compiled from the 25cm-interval topo contours that were generated for the Community Basemap, and in NAVD88 we used nominal 50cm contour for Mean Low Water, and 175cm contour for Mean High Water. Between these elevations one often finds various “water” polygons depending on the application; in our base map we’ll have a purposeful gap.
Along the Sonoma borderlands I struggled for a few hours hand-tracing ponds using NAIP 2009 near-infrared grayscale—the one where standing water is basically black. I was a bit sloppy and found myself tracing at 1:4000 on screen but it was a fairly thorough job through the adjacent shared watersheds along Estero Americano, Stemple Creek, San Antonio Creek, and Petaluma River. I was just about fed up with drawing lines around standing water when I checked in with MarinMap member Bill Voigt of San Rafael for other reasons. He provided an inspired suggestion that I use the water lines from our 2004 photogrammetry. Of course! I had used them in the terrain, but they were not hard constraints and the sparse contouring in west county lands tended to wash over stock ponds in the contours.
Empowered by Voigt’s suggestion, I found that the 2004 photogrammetric water line work, which was only available within Marin County proper, had very high fidelity with 2009 NAIP summer ponds, and it was vastly easier to select the pond-circumscribing lines, sometimes 300 segments at a time, copy and paste them into the Inland_Waters_li features, Merge them all at once into a single compound line, then Explode the multi-parts into discrete ponds. With that approach I was able to harvest hundreds of Marin stock ponds, reservoirs, and vernal pools, and have geometry that appears smooth and accurate at better than 1:2400. There are also quite a few meandering creek features in the west county that are well represented horizontally in the water break lines, but my impression of them was that they took a lot of upgrading to serve as 3D guides in the terrain, and construction rules broke the pond lines wherever a drainage reached its edge. So there were a great many segments that needed to be merged to create closed loops that could build pond polygons.
By the end of this week, we had over 1400 water bodies identified in the watersheds that touch Marin county, and this exercise that was initially motivated by improved cartography may augment our set of candidate sites for wetland inventory. These water features are set into a regional water layer from The National Map that is valid at 1:24k and includes a refined coastline from Big Sur to north of Point Mendocino, and inland to the Sierra crest, from San Luis reservoir up to Mendocino Lake.
We refined Marin and adjacent shorelines (as both MHW and MLW boundaries) from Sears Point, up to and down from Petaluma, along San Pablo, San Rafael, Richardson and San Francisco Bays, the Golden Gate, Gulf of Farallones, Drakes Bay, outer Point Reyes, Tomales and Bodega Bays, and Bodega Head using 25cm interval topographic contours. All Farallon Islands were traced near 1:2400 scale as visible in NAIP 2009 near infrared band.
Next week should find the last couple of administrative layers (Parks and Open Space) and refinements to road centerlines to reflect docks and proper arterial status, and then it will be time to start making test tiles. With any luck, we’ll take some CPU cycles from production server to give us the best shot at getting good tiles fast. With 120 layers, several of which use representations, and many of which use complex Maplex rules for labeling, the Community Basemap Large-Scale Topographic template for ArcGIS 10 is by far the most demanding map document I’ve ever manipulated. Everyone is looking forward to getting the tiles generated for testing, and sending them off to ESRI for review while sending samples to MarinMap members for their comments as well.