Mapping basics

What is a .qgs file?

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.

This might seem like a weird, crappy way to organize things. But it's not. And here's why.

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.

Naming your files and taking mad notes

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.

What is a projection?

* 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.

Demo

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.

The working directory

• Download the data
• Choose your working directory, and stick with it

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

Adding data

We're going to be focusing on VECTOR LAYERS and DELIMITED TEXT LAYERS. These are the layers you will use most often.

Adding vectors

• Click the vector layer button
• Navigate to your working directory
• Add tl_2014_42_county --> tl_2014_42_county.shp
• Let's talk about projections now.
• Add aml_inve --> aml_inve.shp
• Add pa-nursing-homes-20150220 --> pa-nursing-homes-20150220-shp --> pa-nursing-homes-20150220.shp

Adding delimited text layers

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.

• Click the delimited text layer button
• Navigate to your working directory
• Add pa-nursing-homes-20150220 --> pa-nursing-homes-20150220-geo.csv

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.

• Click the delimited text layer button
• Navigate to your working directory
• Add the pa-nursing-homes-20150220 folder --> pa-nursing-homes-20150220-source.csv

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)

The difference between On-the-fly (project-level) projection and layer projection

On-the-fly projecting

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.

Changing a layer's projection

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.

• Right-click the layer you'd like to change
• Select "Save as"
• Click the "Change" button near the CRS section
• Choose your desired coordinate system and click "OK"
• NOTE: this process is going to create a NEW shapefile from your existing shapefile, so make sure to name it something you'll remember
• Name your new shapefile and add it to your working directory with the "Browse" button

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.

Styling

• Right-click or double-click the layer
• Select the Style tab
• Style accordingly

Selecting by location

Now we want to take a look at only those nursing homes that sit on top of abandoned mines.

• Navigate to the Vector menu > Research tools > Select by location

• Select features in = the shapefile you want to refine
• That intersect features in = the shapefile that will determine which features are selected, the intersect layer

• Right-click layer and select "Save as"
• Follow normal saving procedures, but make sure to check the "Save only selected features" box
• You probably want to add this file to the map after it's created, so check "Add saved file to map" also

Analyzing the attribute table

1. Right-click the layer and choose "Open Attribute Table"
2. Navigate to where the layer is stored in your working directory and open the .dbf file with an editor like excel or libreoffice

You mean you want more mapping?

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.