GIS Presentations

Written by charlie   
Thursday, 14 January 2016 19:44
I found the following Open Source GIS related presentations written by GreenWood map over the past 2 years informative, especially the Parcel Mapping complexities found in Wyoming.
  • Skeletonizing Polygons Using PostGIS
  • Cartography and Topology
  • Parcel Mapping WyGEO Meeting
  • Simplifying Geomoetries Using Open Source Tools
The presentations are available on the Greenwood website.
Last Updated on Thursday, 14 January 2016 19:48
 
 

Wetlands in Connecticut

Written by charlie   
Wednesday, 13 January 2016 18:25
Despite being published in 1991, the 130 page compendium, "Wetlands of Connecticut" is a natural primer on the importance of wetlands and how the states relationship and attention towards these important natural features has changed considerably over time. Written by the Fish and Wildlife Service (USFWS) and Ralph Tiner, the author of many wetland related books, it’s a solid reference.

For some background, the USFWS and the Connecticut Department of Environmental Protection (CT DEEP) joined together in 1980 to conduct a wetlands inventory for Connecticut, as a part of the larger National Wetlands lnventory Project. Using aerial photos, almost 173,000 acres of wetlands, equaling approximately five percent of the states land mass, and 86,500 acres of deepwater habitat, including bays, reservoirs, streams and rivers. In comparison, it is estimated that the state totaled 15 to 20 percent wetlands 200 years ago.

Despite differences in the process used to define and identify wetlands between the FWS and the State of Connecticut, in the end, the mapping is generally In comparison with the FWS wetlands definition, the state of Connecticut, solely defines a wetland by soil type, but according to the report, correlate closely on the ground.

Of this total, 18,828 acres of estuarine wetlands, roughly 12,000 acres are considered salt or brackish marsh, of which approximately 8,000 (!) acres have been ditched for mosquito control, generally during the Depression, dewatering entire marshes. I had known many salt water marshes had been ditched, but didn’t realize the extent. After a quick Google search, it appears that Delaware‘s wetland have had a corresponding percentage of coastal wetlands ditched in the familiar grid pattern, and other East Coast states have likely been similarly affected. Almost unnoticed, the report indirectly identifies the results of climate change, and indicates that between Cape Cod and Cape Hatteras, at the time of the report, sea levels have risen 3.5 mm per year; and the period between 1964-1793, saw a larger rise of 4.5mm per year. For contrast, the previously recorded sea rise average of over the last century is 2.5mm/yr, threatening low lying and coastal areas.
 

PostGIS and Raster Data

Written by charlie   
Tuesday, 29 December 2015 17:47
Searching for information on why PostGIS may be be a better option for storing raster data, I came across a presentation "Advanced spatial analysis with PostGIS" by Pierre Racine from the FOSS4G conference earlier this year. The presentation as a whole is chock full of intriguing ideas and the summary below sealed the deal for me on using PostGIS for large raster data sets.

In summary:

Why store raster in the database?

  • One simple language for everything: SQL
  • Many raster functions are similar to vector ones...Complex spatial analyses can be done with a single SQL query.
  • Raster/Vector interaction -Your vector data are normally already in the database...
  • Performance Analysis processing is generally faster on tiled raster coverage.
  • Data volume - You can work on TB raster coverages without much problems.
  • You can even keep the raster’s data outside the database... ...and use them transparently inside the db with SQL
  • Only metadata are stored inside (extent, SRID, pixel type, nodata) Pixel values area read from the referenced files via GDAL.
  • raster2pgsql –R option
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