Sunday, June 25, 2017
Written by Alexandra Kanik (@act_rational)
Revised & presented by David H. Montgomery (@dhmontgomery)
Not all datasets needs to be mapped, but some do! This mapping class is perfect for beginners looking to learn the basics of visualizing geographic data. We'll go over how to find publicly available data, convert addresses to map points, join datasets and use the open source mapping software, QGIS.
Is there a geographical story?
Would a table or chart tell the story just as well?
Am I just mapping where people live?
Analyze geographic data
Create static maps for display
Generate files for interactive map (CARTO or leaflet.js)
Alexandra Kanik tries to keep this mapping resources doc up to date. It could use some reworking and some additional resources, though, so feel free to contribute!
State government websites (such as the Minnesota Geospatial Commons)
College and university research centers (Penn State has this one for Pennsylvania)
IRE Census data
National States Geographic Information Council
Census places - Census places are defined as incorporated places, usually cities, towns, villages, or boroughs. You can read up more on that here.
Your local government should have shapefiles and other data that relate to your local region. Give them a call or visit their website. Here's the GIS office for the City of Pittsburgh.
Peter Aldhous's Refine geocoder - this is nice because it uses geocoders that are, in most circumstances, legal to use. It also geocodes your data using two geocoders so you can compare the results. And it's free! Rate limits may apply.
Census geocoder - Free but I've found it to be a little less accurate. Rate limit: 1,000 addresses at a time.
Here's a more complete list of geocoding tools and APIs
id, city, state, population 001, Pittsburgh, PA, 310885 002, Columbus, OH, 822092TSV
id city state population 001 Pittsburgh PA 310885 002 Columbus OH 822092PSV
id|city|state|population 001|Pittsburgh|PA|310885 002|Columbus|OH|822092
When you save a QGIS map file, it will save as a .qgs. This file is a configuration file. It basically just references the files that you load into QGIS. IT DOES NOT HOLD ANY OF YOUR DATA. This means if you move the shapefiles you load into your map after you save it, your map will break.
So establish a working directory for each map you work on. Put your shapefiles and your map file in there and don't move them around.
QGIS isn't perfect, and neither are you. Sometimes you come up against errors in behavior that you can explain. So your only recourse is to shut 'er down and reopen the program.
And sometimes, that doesn't work either. So you need to scrap your map file.
Now, if all of your work depended on that one .qgs file, that would suck hardcore. But since your shapefiles and data are separate files, and since you've taken crazy good process notes, you're totally fine.
There are going to be projects you work on that will require you to slice and dice a shapefile in many different ways. You need to be careful what you name each file so you aren't analyzing old shapefiles.
I would strongly recommend taking detailed notes about how you change shapefiles so you can return to the notes in case you lose track of which file is which, or if something goes wrong and you need to diagnose a problem.
So the earth is round, right? Turns out it's not the easiest thing to make something round and three-dimensional look flat. But to hell if we don't try.
That's basically what projections are, flat representations of our round world. Lots of people have taken a stab at making the best projection, but not all projections are created equal.
Depending on the scope and span of your data, you may want to stick to a more local, granular projection. But if you're trying to show the whole world things are going to get a little wonky at some point.
* I've collapsed this section because while it's very important to understand projections, they are also amazingly confusing and might serve only to confuse the novice map maker. Just know to return to this topic once you feel more comfortable with QGIS and displaying and analyzing geographic data in general.
In this demo, we're going to be creating a map that overlays Pennsylvania nursing homes with abandoned mine data.
This is a blank map.
A working directory is a place on your computer where you house all of your map shapefiles. Once a shapefile is added to the working directory, it should not be moved around because that can break the file path and therefore your map.
In the example above,
abandoned-pa-mines-data is the working directory.
This is what you'll see if you move layers and map files around
We're going to be focusing on VECTOR LAYERS and DELIMITED TEXT LAYERS. These are the layers you will use most often.
There are two types of delimited text layers you can add: geographic and non-geographic.
Geographic delimited text layers
So we've already add the
pa-nursing-homes-20150220 as a shapefile, but there's another
pa-nursing-homes-20150220 data file in our working directory.
On the data import screen, we have many options. Most of them are self explanatory. But look under Geometry definition. This is where we will be telling QGIS that we have a delimited text layer that has geographic data.
Make sure Point coordinates is checked.
Then set your latitude and longitude fields
Write this down:
Latitude = Y
Longitude = X
Non-geographic delimited text layers
If your delimited text file doesn't have those latitude and longitude columns, you can still import the data, but nothing will be mapped
This is often useful and necessary when you're trying to join non-geographic data to geographic data. Michael Corey goes over that in his presentation.
Because we have no geometry in this file, QGIS won't let us add it to the map until we tell it that there's No geometry (attribute table only)
On-the-fly projecting is a way of setting the project-level projection so that subsequent shapefiles added to the project assume that specified projection.
You'll want to set this on-the-fly projecting before adding any data to your map.
NOTE: On-the-fly projecting does NOT actually change your shapefile's projection. So next time you use this shapefile in a new map, it will revert back to whatever projection it had before you added it to your on-the-fly map. If you want to actually change a shapefile's projection see this section.
If you'd like to change a layer's projection instead of just changing how the layer looks in the QGIS system, you'll need to follow these steps.
You might be thinking... well that didn't seem to change anything. But close down QGIS, open a new map, and add that newly projected shapefile. You'll see that it now looks much different. Much more like Pennsylvania is expected to look.
Now we want to take a look at only those nursing homes that sit on top of abandoned mines.
So you've got a nice map here. What can you do with it?
There are ways to get from QGIS to an interactive map, but we're going to focus on something quicker: how to export a static image of your map.
There's two ways: one easy, and one more complex but powerful:
Go to qgis.org to download the mapping program yourself.
This is only the very beginning of what's possible with QGIS. Here's a link to Michael Corey's presentation where he covers joining datasets within QGIS, simplifying shapefiles and grouping points by location.