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Archive of posts filed under the Viz category.

Five Colors for Stats

I am building some visualization layers in Geoserver from PostGIS, which requires .sld files (until Geoserver catches up with the CSS styling world – oh wait, look here). It is convenient to show ranges using ColorBrewer2 colors in a set of one plus five.. a color for NoValue, then what I call little0, little1, central […]

Earthscapes USPS

Slice a Polygon by a Grid

Sometimes there are polygons with too many vertices to process comfortably. For that or other reasons, you may want to cut one or more polygons with a reference grid. Here is an example using the US 2010 Census and a set of grid cells in EPSG:3310: ——————————————————————– — take a single census place polygon and […]

PostGIS 2.0 Intro Presentation

at UC Berkeley Soda Hall, for the San Francisco PostgreSQL Users Group and again at the College of Natural Resources for GIS Day 2012 main slides and dustymugs’ raster slides errata: * The ST_MakeValid(geom) example could have been expanded to include ST_IsValidDetail() * It is arguably wrong to say that Lat/Lng (EPSG:4326) data is one […]

Calling Bluff

Two years ago a hard disk arrived on the desk of a colleague, from persons claiming to have some kind of parcel data “for every state in the US.” Naturally, being very skeptical and at the same time, just a bit eager to show off open source tools on linux, I whipped up a script […]

Grid 150m classification

This visualization shows two quantitative dimensions at once, on a 150 meter square grid in a study area in California. The base color is a category of land use, and the gray shading is a percentage of developable acreage. The number on each grid cell, drawn using QGis, is synonymous with the gray shade.

Stage of Summarization

Here grid cells are classified into three types by landuse, and county parcels are split along grid cell boundaries, in preparation for computing a result stored into a database table of grid cells as table rows. Note that roadways, water bodies and some public lands are correctly absent in net calculations, but since the area […]

Local Blockgroups 2010

As a way to learn about the geo constructs in the U.S. Census data, I built a few visualizations of the area where I live.     Elementary Constructs  The heavy green lines are census tracts, with their tracts ID shown in large. The pink lines are census block groups. Census blocks are shown, along […]

PostGIS Geocoder behavior

I became interested in the practical behavior of the     PostGIS TIGER 2010 Geocoder when I started doing statistical profiling of the results of geocoding 100,000 standard addresses in my county and got results that looked like what you see in the image at right. That inquiry morphed into this short paper PDF. Why statistical profiling? […]

ST_DWithin on a Grid

As part of a larger inquiry, I wanted to know how ST_DWithin() behaved on a regular grid – that is, what is the relationship of the input radius, and the number of grid cells that could be selected in the result. The manual implies that it is an intersects relationship not a contains relationship, and […]