Georeferencing Maps and Creating Shapefiles

Bogdan G. Popescu

bogdan.popescu@johncabot.edu

John Cabot University

Today

Today’s agenda includes:

Georeferencing Maps
Creating shapefiles based on georefrenced maps
Using QGIS - free software that we will use to creat shapefiles

Downloading QGIS

To download QGIS go to: https://qgis.org/en/site/

Click “Download Now”

Then choose QGIS for MacOS or Windows depending on your operating system

Note that this is a large file (approx. 1.7 GB)

Installed program will be 3 GB

Installing QGIS: MacOS (left) vs. Windows (right)

Opening QGIS

Case: Republic of Serbian Krajina

Let us examine a historical case.

The Republic of Serbian Krajina was a territory within the independent Republic of Croatia.

It was active during the Croatian War of Independence (1991–95).

The RSK government waged a war for ethnic Serb independence from Croatia.

Case: Republic of Serbian Krajina

This information is from:
https://en.wikipedia.org/wiki/Republic_of_Serbian_Krajina

Case: Republic of Serbian Krajina

The picture itself is available at:

https://upload.wikimedia.org/wikipedia/commons/6/61/Map_of_Republika_Srpska_Krajina.png

Georeferencing

We will thus try to georeference this map

Georeferencing - the process of taking a digital image (airphoto, a scanned geologic map, or a picture of a topographic map), and adding geographic information to the image

The goal is that a GIS or mapping software can ‘place’ the image in its appropriate real world location.

Georeferencing

Digitizing the Map

Now we would like to create a polygon out of the red area

Digitizing the Map

This is what our data looks like:

Digitizing the Map

class(occupied)

[1] "sf"         "data.frame"

Digitizing the Map

#Step1: Loading the data
occupied <- read_sf(dsn = "./data/occupied.shp")
head(occupied, n=3)

Simple feature collection with 2 features and 1 field
Geometry type: POLYGON
Dimension:     XY
Bounding box:  xmin: 15.28861 ymin: 43.73483 xmax: 19.4268 ymax: 45.88013
Geodetic CRS:  WGS 84
# A tibble: 2 × 2
     id                                                                 geometry
  <dbl>                                                            <POLYGON [°]>
1     1 ((15.42635 45.33129, 15.39042 45.09504, 15.40239 45.23016, 15.40239 45.…
2     1 ((18.42669 45.73402, 18.37878 45.68802, 18.37878 45.68802, 18.37878 45.…

Digitizing Maps

As you might imagine, we can use multiple maps at the same time to create shapefiles

Digitizing Maps

As you might imagine, we can use multiple maps at the same time to create shapefiles

Conclusions

We have learned how to make shapefiles and how to georeference maps

QGIS is a free software that allows us to digitize maps.

The shapefiles that we create can be later loaded onto R and used for spatial analysis