May 25, 2011

1-2-3: Mirror Modelling in Inkscape

The other day, I needed to draw a symmetrical object (a map point icon) in Inkscape, the wonderful free vector graphics editor. I was surprised to find that there is no symmetrical modeling feature, so I improvised. Here’s how to create a symmetrical polygon in Inkscape:

Draw half of the object, using the line tool (I know you want a polygon, but draw one side of the object as a line). If you want something that is symmetrical both horizontally and vertically, draw one quarter of the object. I suggest using a guide, so that you can get the ends of the line to be vertically aligned (for vertical symmetry, i.e. left and right) or horizontally aligned (for horizontal symmetry). Get the shape right, because once you create the polygon, the sides will no longer automatically mirror each other.
Copy the line you just made and paste (ctrl-alt-v, Paste in Place, may make it easier to line up later, although it will be invisible when first pasted because it will align perfectly with the original path) it into the document. Choose Object->Flip Horizontal or Object->Flip Vertical, depending on the axis of symmetry. Move the flipped object so that the ends line up (i.e. it forms a polygon). You can use the arrow keys to align it; holding Alt while using the arrow keys makes them move the object finely. If you are doing two-axis symmetry, create 3 copies and flip one horizontally, one vertically, and one both vertically and horizontally, then line it all up.
Use the paint bucket tool to fill the area contained by the lines. This will create a polygon that is symmetrical. You can now either delete the polylines or move them to a hidden layer if you think you'll need them again.

April 21, 2011

Blog to Watch: Under Dark

If you’re interested in Open Source GIS and especially QGIS, I highly suggest you give the blog “Under Dark” a read - the author seems to be very clued in the latest developments, and the blog is very active.

April 19, 2011

Getting Started with Open-Source GIS: Primer

I was talking to some friends at school tonight, and I realized what a barrier it can be to actually get some of the open-source geospatial tools I’ve talked about on this blog going. So I’m going to start a series about how one can get started with these tools, from a level-zero technical knowledge (of course, I assume basic GIS knowledge—if you know what I mean when I say buffering, geocoding and vector analysis, you’re fine). I’ll be approaching it from the standpoint of a Windows user (although OS X users with Intel Macs should be able to follow the same instructions, and if you’re a Linux user, you should have no problem).

April 6, 2011

Basemaps in QGIS

**Update 2012-02-02 17:57 -0800:I just wrote a post about another way to do this.

I really like QGIS. It’s a powerful GIS that runs on Linux (among other operating systems) and doesn’t require incredible amounts of CPU. One criticism I had of it was that it did not provide the same ‘click-and-go’ basemap experience one can get with ArcGIS for making simple maps (i.e. Add Basemap, select Bing, DeLorme, &c. and you’re done). No more! Thanks to the qgis-openlayers plugin available on GitHub, you can now use OSM, Google Maps, Yahoo Maps and Bing Maps as base layers in your project (side note: make sure you don’t violate any copyrights by using them). ~~I’ve had a little trouble with the projections, but I’m not that good with dynamic reprojection in QGIS yet, so I’m sure it’s my error.~~; Unfortunately, the layers don’t support reprojection, so your project is basically forced to use Google Mercator—which may be a showstopper but is often acceptable for quick maps (remember, QGIS can reproject your other layers). If you do want reprojection, see this post.

April 2, 2011

Plug: Google Fusion Tables

Google Fusion Tables is a fairly new service many may not have heard of: it lets you manage large tabular datasets in the cloud, using Google’s infrastructure. You can then visualize that data in a variety of ways, including a Google Map. The coolest feature in my mind is the ability to geocode addresses to a map on the fly. There are lots of datasets out there that, rather than providing a mappable latitude and longitude, provide an address column (or street, city, state, &c., which is easily turned into an address column using the concatenate function of your favorite spreadsheet)–many of the datasets on DataSF are like this, as are many others.

April 2, 2011

Agencies Providing Real-Time Data

Note 2012-06-15: this page is now quite outdated.

As a part of the Transit Appliance Project, I’ve been evaluating what transit agencies have real-time data; here’s a (non-exhaustive) list, focused primarily on U.S. agencies (note: these are only agencies that provide an API, not any agency with a real-time feed):

TriMet, Portland, OR, custom JSON $- P$ feed, XML may also be available. GTFS-realtime is also available.
San Francisco Muni, via NextBus.
Bay Area Rapid Transit (BART), San Francisco, CA, custom XML feed. Their Google Group is very responsive and helpful. GTFS-realtime is also available.
AC Transit, Oakland, CA, via NextBus.
Los Angeles Metro, both via NextBus and a custom XML API.
Unitrans, Davis, CA, via NextBus. Their GTFS is hosted by Sacramento RT.
MBTA, via NextBus. They have a GTFS at http://www.eot.state.ma.us/developers/ that has stop ids to match NextBus. They have a trial up for rail arrivals.
Transport for London (TfL), custom XML API.
Washington Metropolitan Area Transit Authority (WMATA), custom XML/JSON $- P$ API; has real time arrivals for rail, but only positions for buses (no arrivals).
DC Circulator, via NextBus. I couldn’t find any public GTFS.
Chicago CTA, custom XML API for buses, no arrival data for trains yet.

I know that there are a ton that aren’t on here; as I do more research, I’ll put more up.