March 3, 2012

Conditional Labels in QGIS

I fairly commonly find myself in a situation where I would like to display one label for certain features and another for other features in the same layer. QGIS doesn’t have an official way to split labels up into categories, and until now I’d resorted to having two layers to render otherwise identical features. But, in the new, excellent expression based labeling from Nathan Woodrow, I realized one can use an SQL CASE statement.

February 2, 2012

More Basemaps in QGIS

One of the more popular posts on this blog has been my piece on adding basemaps to QGIS. While the OpenLayer plugin is great, one of the things that I find dissatisfying is that it requires reprojecting your data to match the EPSG:3857 basemap. I often work in State Plane, and I’d just as soon have my data stay in that projection, which will also minimize local distortion. Well, as it turns out, one can add tiled map services as GDAL raster layers, with all the benefits that entails (e.

December 30, 2011

Transit to Everywhere

Data courtesy MapQuest and OpenStreetMap CC-BY-SA, the City and County of San Francisco, and Bay Area Rapid Transit This is an overlay of the transit and walking trip plans generated by OpenTripPlanner from Powell and Market to every other intersection in San Francisco, after Eric Fischer’s map of walking routes to every intersection in San Francisco. It brings out the transit routes but also shows well-used walking routes. The lines do not vary in width (don’t let Market Street fool you, it’s actually several lines—BART, MUNI rail in 2 directions, Muni bus, walking—very near each other).

December 23, 2011

GitHub Image Diffs

As you may have gathered, I like Git and GitHub. Today, I ran across a GitHub feature that is really cool and above and beyond the call of duty: not only do they produce and display diffs on text files, but also on image files! You can see an example in one of my repositories. Added points if you can figure out where the map tile is from!

December 13, 2011

Shapefiles in OpenLayers

Update 2011-12-14: It seems that a lot of people are coming here from web searches with phrases like “shapefile openlayers.” If all you want to do is display your data in OpenLayers, I’d highly recommend using a program like Quantum GIS to convert your Shapefile to a more web-friendly format like KML or GeoJSON. Both of these formats can be read by OpenLayers directly, and you’ll see faster performance and more browser compatibility than if you were to load your Shapefiles directly.

November 12, 2011

Another LA Metro Visualization

Here’s another visualization of the data used in the previous post; I made the lines a lot finer, so the noise is less visible. It’s easier than ever to see the Silver Line. I classed the data manually this time.

November 12, 2011

Making Transit Travel Speed Maps with Open Source GIS

Update 2011-11-12 8:21 -0800: I just posted a visualization I like better. The Internet has been abuzz the past week regarding transit speed maps. It seems to have been spurred by a post on Bostongraphy, which was inspired by many of the amazing visualizations produced by Eric Fischer, especially this one. Indeed, this blog has gotten a fair bit of traffic itself, because Andy Woodruff of Bostonography used my avl2postgis project to retrieve the data.

October 29, 2011

Google Maps Tile Scales

I found this buried deep in an appendix of the Mapnik XML Schema Reference, and I thought it so useful I am reposting it here: Zoom level Scale denominator 0 559,082,264 1 279,541,132 2 139,770,566 3 69,885,283 4 34,942,642 5 17,471,321 6 8,735,660 7 4,367,830 8 2,183,915 9 1,091,958 10 545,979 11 272,989 12 136,495 13 68,247 14 34,124 15 17,062 16 8,531 17 4,265 18 2,133 19 1,066 20

September 8, 2011

Mapping Real-Time Delays: Review

Some readers may have noticed that I’ve updated my last post several times in the last few days. After thinking about the algorithms I used, I realized there were some significant issues with them. I’ve explained them a certain amount in my updates to my previous post, but I’d like to expand on the issues a bit here. Using an Inverse Distance Weighting algorithm exaggerates delays where stops are sparse by allowing them to spread over larger areas; the graphic should make this clear; if the red dots are stops with delays, one in the city center and one in a suburb, it is clear that the delays will be magnified where stops are sparse (figure 1), because there are less stops around it.

September 5, 2011

Mapping Real-Time Delays

Update 9/8/2011: I’ve posted a short piece about the significant biases and issues with this visualization. TriMet delays on the evening of 9/5/2011. They cleared up the large delay east of Portland not long after I took this screenshot. GTFS-realtime was released a few weeks ago, and I thought the time was right to create a visualization of the data. This project is an OpenLayers web map (pictured at left, click for larger version) using OpenStreetMap data (and the MapQuest-OSM Tiles), which is overlayed with a raster showing areas that are currently being affected by delays.