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!
