In this tutorial I'll be showing you how to do basic georeferencing using the software QGIS.
This image is what we will be making today.
To get started open QGIS.
There are a couple plugins that we will be using to make sure that you have them enabled.
In the top bar, go to plugins.
manage and install plugins.
The two that we'll be using are open layer and Georeferencer GDAL. Let's start with open layer.
As I look down in the menu here I can see that the open layers plugged in-- plugin is both installed and enabled.
So next I will search for Georeference GDAL.
Here, that is both installed and enabled.
Now that I've made sure both of my plugins are enabled, I'm ready to begin.
I'll go to web.
open layers plugin
open street map and open street map.
Open street map is an open source version of Google maps.
This is a satellite image map. Now in order to ensure that I have the correct
coordinate reference system, I'm going to change it in the properties.
I'll go to project
to project properties
and select WGS 84 pseudo mercator
hit apply
and OK
Now I want to zoom in on my target.
Since we will be overlaying a map of historic RIT's campus and current RIT's campus, we'd like to go to RIT.
You can click on a point on the map to zoom in on it or to locate your target.
To save some time I've located RIT's coordinates in advance.
And I'll change the scale.
to one
to 25,000 so that we can get a closer view.
Now that we have our
close-up map of RIT, we can overlay our historic map over top of it.
In the top menu go to raster.
A raster image is a bitmap image.
You can think of it as an image file.
In the raster menu go to Georeferencer to Georeferencer
This opens the Georeferencer plugin.
Go file
open raster to select the image file that you would like to overlay.
I'm going to select Henrietta plot 26.
Which is the historic location of RIT's campus.
We need to make sure that our coordinate reference system matches the one we used for open maps.
So select WGS 84 pseudo mercator.
and then click OK.
Now that it's loaded in our image.
We're going to add points that correspond on this image
to points that correspond on the open layers map.
For example this intersection
exists in both this map and the satellite map.
So we're going to click add point.
and click the intersection.
Tt gives us an opportunity to manually insert our coordinates, or we can click map from canvas and simply click the intersection.
And it will auto fill these based on the coordinates of the point we clicked.
Now I can hit OK.
These points will be used to correctly overlay your historic map over top of your new map.
Ideally you would have four points
in a rectangle with potentially a fifth point to mark the center.
to save time I'm going to load points that I've prepared in advance.
Now that my points are selected I need to adjust my transformation settings. These are the settings
that the georeferencer will use to map our historic map to our current map.
In settings go to transformation settings.
The transformation type should be thin plate spline.
The resampling method should be nearest neighbor.
The target SRS should be selected CRS CRS stands for coordinate reference system.
For output raster you are going to designate the name of the file that you want to save to.
And hit save. This is the pathway that it will save
the modified version of your map to.
Make sure the compression is none.
Zero transparency when needed
and load in QGIS when done.
Then hit OK.
That we set our transformation set-- settings
we're ready to begin.
Up in the top click start start georeferencing.
It will give a notification that the raster was successfully generated.
We will mimize our georeferencer
to see appear. If we zoom out
we can see it how the map has been lined up
with the open street map.
How this feature of the river is continuous along both of them.
Now if you wanted to see features from both maps simultaneously
there's a couple different approaches.
Most people's instinctive approach is to simply add transparency
to the historic map.
I'll show you what that looks like.
In your raster map right click
and go to properties.
Under transparency you can adjust the global transparency. Let's try 40%.
And then hit apply.
Now you can see there added transparency, but you can't quite make out features of either map.
I advise against using transparency as it seems to only obscure features of both.
So instead.
I'll return the transparency down to zero.
and hit apply.
Then hit OK.
What you can do instead is mark the features on a new shape layer,
and then hide the raster map so that the features from that map are visible without the entirety of the map.
I'll show you what I'm talking about.
Under layer go create layer.
New shape file layer.
The type will be polygon.
Remember, we're intending to show the relationship of farms on Henrietta plot 26 to RIT's
current campus. So polygons will allow us to trace the outline of the farms.
This field list allows you to give attributes to your shape layer.
Right now it's an integer which would allow us to uniquely number each of our farm polygons.
This isn't as useful as naming them.So I will remove this field,
and instead add a new field that is
farm name.
It is text data with a length of 80 characters and I will add it to the fields list.
Now we have an available attribute to name our farms.
We'll hit OK.
Now we must name our shape file layer.
I will call it
Henrietta farms
and hit save.
Now
I can go up to the top
and click toggle editing.
I can then click
add feature
and trace the outline of a feature that I would like to show.
Right now I'm tracing one of the larger farms, the Corbin farm.
When all my points are in place I can right click to complete the shape.
Now the field that we generated earlier, farm name is available to input data into.
I will name it Corbin
and hit OK.
Now we have our polygon laying over top of both maps.
This is useful because now we can hide
the historic map and still see the relationship of the Corbin farm to RIT's current campus. As you can see it's pretty much the whole thing.
There are some styling options that you have when dealing with the shape file layers.
If you go over to your layer styling menu
and change it from single symbol
to categorized.
You can individually style each of your polygons.
It appears that it's disappeared
but it simply isn't rendering the way that you expect it to. To cause it to reappear, simply click
classify.
It will automatically generate a null symbol, you can simply delete it.
If you'd like to show the label for this.
You can go to labels which is right below the styling menu.
In the drop down menu for labels go to show labels for this layer
and under label with select the attribute that has the name.
Then hit apply.
In order to add a different styling to a different polygon.
You'll need to save changes to your layer edits
and then create another farm.
Hit OK and then click classify.
When you're ready to export to an image
simply make the map appear as you want the image to.
For instance if I were not interested at all in seeing this historic map and simply wanted to
see the relations between the farms in the campus I could deselect the historic map
and when I go to project
save as image
I will give it a title, I'll name it campus with
farms
and hit save.
Now if I navigate to the directory
that I've saved it to and open it
we can see it appears exactly as my screen did.
There are many more options with QGIS, this has been simply a basic overview.
Happy Georeferencing!