Tuesday, May 28, 2013

TileMill and MapBox

Publishing a map on the web can be accomplished through many different ways and programs.  Costs, ranging from free to expensive, are mostly based on the popularity of the map (i.e. number of views) as well as how much information is stored to create the map.  Performance of the map can also be a key component to cost.

MapBox, a website, and it associated downloadable tool, TileMill, are a great way to create a visually appealing, interactive, and quickly deployable map.  The fully interactive map is available here. The map below works correctly in Firefox.

A screenshot below shows the TileMill program with an example of easy coding on the right-hand side.  If you decide to give TileMill a try, be sure to check out its Crash Course!
Screenshot of TileMill

Saturday, May 18, 2013

Census Data: Easier to Use

The Census Bureau has come a long way by offering census data in formats that can be easily imported into GIS software.  Whether at small or large scales, census data are vital to any analysis.  Of course, census data are free, even though some companies charge!  In addition, it is noteworthy to add census data can be ordered on DVD and includes user friendly tools to help extract the data you need.

Previously, and in still in some cases, attribute data would have to be joined with shapefiles.  The TIGER/Line page now features demographic and social data pre-joined to shapefiles and geodatabases for users that are not familiar with joining and managing such complex data. Click the map below to enlarge it.

US Population Counts by County and Cities with Population Greater than 250,000

Data from the American Community Survey (ACS) 5-year estimates can also be downloaded easily.  However, more can be done and more ready-to-use files could be created--resources allowing!  Hopefully, the Census will be able to maintain what they are doing and expand in the future.

Family Size (Purple/Red = Greater than 1 Standard Deviation above the mean, Blue = Below, No Color = Mean to 1 SD Above).  Both maps are derived from data in the Summary Demographic Profile 1.

Maps made with QGIS

Sunday, May 5, 2013

Space-Time Cluster Analysis with SatScan

For more information: Visit the latest post on SatScan: http://opensourcegisblog.blogspot.com/2015/02/satscan-94-released-better-than-ever.html

Original post
Numerous basic and advanced techniques exist for finding spatial and temporal clusters.  Searching for clusters has broad applications for any field of scientific inquiry!

Unlike other spatial models in other free and paid software, SatScan's statistics' probability distributions allow for poisson (count data and rates) and binomial distributions--to name two.  There is also the ability to treat same data as continuous.  You won't find an easier way to do this than with SatScan!

SatScan is a free program but requires several steps to get data into it for analysis.  For most analyses you will need three files in a text delimited format -- without column headers (such as variable names).

The three files: 1)  A case file with a column for the geographic unit. day, month or year (see documentation), and number of cases.  You can aggregate the data into any geographic unit--large or small. 2) A geographic coordinate file (cartesian or lat/long) with the name of the unit (i.e. census tract), x and y for centroids of the geographic units, and 3) population file with the estimated population over the time period-- by year.

After this slightly painful process, which one learned, can easily be duplicated, one can easily perform complex spatial analysis and adjust key parameters such as the population at risk and maximum size of the cluster.  Time units are important, and you will have to make key decisions as to how long a cluster may have to develop--depending on the problem of interest.

SatScan can look for purely spatial, purely temporal, space-time, and spatial variation in temporal trends in data.   SatScan uses 'scan' statistics/scanning window and cylinder to finding and differentiating potential clusters.

SatScan's output includes *.txt and/or *.dbf files of the results and clusters.  The *.gis file can be joined to the shapefile of the geographic units, which are using, to show risks and different clusters.  This part is straightforward and less painful.  You will need to take your time selecting parameters and interepreting results!

Two good articles to read are: 1)  Block's Tutorial and Review   and 2) Visual Analytics of Space-Time Statistics.  The SatScan manual on its website also has a great list of references.

Additional Article: