Archive for the ‘Maps’ Category

Mapping ACS Census Data for Urban Areas With PUMAs

Tuesday, December 16th, 2008

The NY Times wrote a story recently based on the new 3 year ACS data that the Census Bureau released a couple weeks ago (see my previous post for details). They created some maps for this story using geography that I would never have thought to use.

Outside of Decennial Census years, it is difficult to map demographic patterns and trends within large cities as you’ll typically get one figure for the entire city and you can’t get a break down for areas within. Data for areas like census tracts and zip codes is not available outside the ten-year census (yet), and large cities exist as single municipal divisions that aren’t subdivided. New York City is an exception, as it is the only city composed of several counties (boroughs) and thus can be subdivided. But the borough data still doesn’t reveal much about patterns within the city.

The NY Times used PUMAS – Public Use Microdata Areas – to subdivide the city into smaller areas and mapped rents and income. PUMAs are aggregations of census tracts and were designed for aggregating and mapping public microdata. Microdata consists of a selection of actual individual responses from the census or survey with the personal identifying information (name, address, etc) stripped away. Researchers can build their own indicators from scratch, aggregate them to PUMAs, and then figure out the degree to which the sample represents the entire population.

Since PUMAs have a large population, the new three-year ACS data is available at the PUMA level. The PUMAs essentially become surrogates for neighborhoods or clusters of neighborhoods, and in fact several NYC agencies have created districts or neighborhoods based on these boundaries for statistical or planning purposes. This wasn’t the original intent for creating or using PUMAs, but it’s certainly a useful application of them.

You can check out the NY Times article and maps here – Census Shows Growing Diversity in New York City (12/9/08). I tested ACS / PUMA mapping out myself by downloading some PUMA shapefiles from the Census Bureau’s Generalized Cartographic Boundaries page, grabbing some of the new annual ACS data from the American Factfinder, and creating a map of Philly. In the map below, you’re looking at 2005-2007 averaged data that shows the percentage of residents who lived in their current home last year. If you know Philly, you can see that the PUMAs do a reasonable job of approximating regions in the city – South Philly, Center City, West Philly, etc.

The problem I ran into here was that data did not exist for all of the PUMAs – in this case, South Philly and half of North Philly had values of zero. According to the footnotes on the ACS site, there were no values for these areas because “no sample observations or too few sample observations were available to compute an estimate, or a ratio of medians cannot be calculated because one or both of the median estimates falls in the lowest interval or upper interval of an open-ended distribution”. So even though the PUMA geography is generally available, there still may be cases where data for particular variables for individual geographies is missing.

Just for the heck of it, I tried looking at the annual ACS data which is limited to more populated areas (must have 65k population where 3 year estimates are for areas with at least 20k) and even more data was missing (in this instance, all the areas in the northeast). Even though PUMAs have a minimum population of 100k people, the ACS sampling is county based. So even if the sample size for a county is ideal, they may not have a significant threshold for individual places within a county to compute an estimate. At least, that’s my guess. Regardless, it’s still worth looking at for the city and data you’re interested in.

ACS Data for Philly Pumas

Red States / Blue States

Saturday, August 23rd, 2008

It’s been a busy summer – I’ve spent a good chunk of it working on an election mapping project. The library wanted to create a resource for students to use for the upcoming 2008 presidential election. Here it is:

Red States, Blue States: Electoral Strategy Behind the Map

A few of the procedures and issues I encountered while working on the project became fodder for a number of posts to this blog, so I thought I’d share the end result.

I’ve also been assembling data and pages for a server I’ve been given to provide GIS data at Baruch, and have been investigating open source alternatives to ArcGIS. More on that later…

Image Formats for Exporting Maps

Wednesday, June 11th, 2008

I’ve been working on a project where I need to create maps in ArcGIS, save them as images, and embed them in a webpage. Seems simple enough right? Well, it turned into a much more complicated affair, as the file formats I was using to export the images looked terrible. I thought I would share this experience, as I had a hard time finding info about it and I imagine this is a problem that many have faced at one point or another.

I was exporting some basic two-color thematic maps of the US out as jpegs, and the colors were blurry and the boundaries block-like. I tried increasing the resolution, which didn’t work because it made the images larger. Couldn’t do that, because I needed the images to be a specific size to mesh with the content on the pages I was creating. So I tried tiffs and gifs as well, which were only mildly better.

I recalled having these problems in the past, but I always got around it by exporting the maps out as pdfs, which look pretty good. But in those cases I was just trying to preserve the map in a static format, and since you can’t embed pdfs into html (you can only link to them) that option was out. I’ve used emf files when my goal was to insert the image into a Word document, but emfs are not recognized by web browsers nor can they be embedded in html, so no dice there.

As I delved into this further, I discovered that pdfs and emfs looked good because they are vector based. Since the map I was creating is vector based, the conversion is pretty clean. The jpegs and tiffs are raster based. So when you make the conversion, the image quality suffers, particularly when using jpegs as the files gets compressed. So, what I really needed here was a vector based image format that you could view in a web browser.

This is when I stumbled upon svg files – scalable vector graphics. They are open standard, vector based, and are essentially xml files. You can even open them in a text editor and, if you know what you’re doing, edit them. They are scalable because you can zoom in and out without the resolution getting poor. SVG files can be viewed using recent versions of the Firefox broswer, and you can embed them into html using an object or embed tag (can’t use the standard img tag). They look great – crystal clear. The problem here is that Internet Explorer doesn’t support svg without a special Adobe plugin. Doh! Which means if you’re designing a web page with svg files, only 18% of the web surfing population can view them without having to bend over backwards. So, that’s not going to work.

Then I was surfing around Wikipedia (for unrelated reasons), and noticed that several maps embedded in their pages are in SVG format. And, I was able to view them in Firefox and IE without a problem, and without a plugin. Then I discovered on one of the documentation pages that they use a program within the MediaWiki software called RSVG that draws from a library called librsvg, which rasterizes all of their svg files. The program looks like it does a great job. But getting the web server I’m using configured to handle this is beyond my control. But it’s good to know that there is a server-side solution.

I did find a detailed page on Wikipedia that was created to guide people in submitting images to the site, and they recommended using SVGs or PNGs – portable network graphics, which are an open standard raster format. They had some useful illustrations comparing the quality of the different images and the reasons why some are better than others.

In the end, I went with the png format, which still isn’t as crisp as the svg but is far better than the other rasters. And, it’s widely supported, so no problems embedding it in html with the standard img tags. Some older versions of the IE browser may render them a little funny, or not at all, but you’re safe if you’re using version 6 or 7. Hopefully, the next version of IE will support svg, as it does provide some great opportunities for creating maps outside of GIS. If you have an svg file with countries of the world (download one for free from wikipedia), you can open it in a text editor and assign different countries different background colors based on a range of values. But that’s another story.

In summary, when you want to save maps as static files:

– Use pngs if you want to embed them in a web page
– Use emf if you want to embed them in a word processing document
– Use pdf to preserve the map in a stand-alone file for linking to or printing
– Use svg for preserving stand-alone maps for viewing locally or printing

Compare:

JPEG Map vs PNG Map

(You can download an SVG example as well and take a look in Firefox. For some reason, if you try and view it directly from here, you’ll see the code and not the image – may have something to do with the configuration of this webserver – I’ll have to investigate).

Useful links:

– Wikipedia guidelines for submitting images, includes discussion of jpeg, png, svg
– Wikipedia guidelines for svg
– Download svg map images from Wikipedia
– Instructions for embedding svg into html
SVG homepage
PNG homepage
Adobe SVG viewer


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