Author: Ketty, Ben Hur
This module is designed to teach you how to use QGIS tools to create a new GIS layer. You’ll learn concepts like digitizing and georeferencing. In addition, you’ll learn the following concepts;
Let’s start with an example:
Let’s imagine that your department is planning to build new hospitals for the province. They may want to conduct an environmental pre-assessment for the planned infrastructure project. The assessment may involve conducting research on available data. As you might imagine, there is a great deal of geographic data available in formats that can not be immediately integrated with other GIS data. Some of this data may be in the form of old maps, high resolution satellite imagery or remotely sensed imagery which is often used for making digital maps like Google Maps or OpenStreetMap. Creating and Editing is good for updating changes to your layers or database. This is where Creation and Editing of layers comes in. One way in which new data can be created in a GIS is through digitizing. Other methods include georeferencing, scanning and vectorization, clipping, selecting and then saving. It’s important to remember that the created data is dependent on the uniqueness of the original source. For example the image below shows that each image product will produce a unique dataset.
Figure 7.1: Remotely sensed images of different coastlines. A. Interferometric Synthetic Aperture Radar (IFSAR) data (using microwaves to collect data for creating representative images), B. topographic and bathymetric lidar data (height and elevation data represented by colors), C. hyperspectral imagery (adding color to invisible energy), and D. digital photography (human eye visible colors represented as themselves)
Digitizing in GIS is the process of “tracing”, in a geographically correct way, information from media/maps. The process of georeferencing relies on the coordination of points on the scanned image (data to be georeferenced) with points on a geographically referenced data (data to which the image will be georeferenced). By “linking” points on the image with those same locations in the geographically referenced data you will create a transformation that converts the location of the entire image to the correct geographic location.
Figure 7.2: Here, buildings have been digitized (traced) on top of an aerial photo
Georeferencing, on the other hand is the process of taking a digital image, it could be an airphoto, a scanned geologic map, or a picture of a topographic map, and adding geographic information to the image so that GIS or mapping software can ‘place’ the image in its appropriate real world location.
QGIS allows you to create new layers in different formats. It provides tools for creating GeoPackage, Shapefile, SpatiaLite, GPX format and Temporary Scratch layers (aka memory layers). Editing allows you to add, delete and modify features in vector data sets. The first step is to put the data set into edit mode. Select the layer in the Layers panel and click Layer | Toggle Editing. Alternatively you can right click on a layer in the Layers panel and choose Toggle Editing from the context menu. Multiple layers can be edited at a time. The layer currently being edited is the one selected in the Layers panel. Once in edit mode the Digitizing Toolbar can be used to add, delete and modify features. |
Digitising, as you might have guessed, is the art (or science) of creating digital vector data from another source, such as a raster image. In order to begin digitising, we must first enter edit mode. GIS software commonly requires a separate mode for editing, to prevent users from accidentally editing or deleting important data. Edit mode is switched on or off individually for each layer.
Figure 7.3: New GeoPackage Layer dialogue
Specify the coordinate reference system using the button
To add fields to the layer you are creating:
Figure 7.4: New shapefile layer dialogue
Specify the coordinate reference system using the button, you may select the universal WGS 84 which is well suited for web mapping projects across the globe
This tutorial will show how to create a new shapefile using auxiliary data like satellite imagery provided by Google. In practice accurate ground truth data will be available. This would be accurate qualitative data about the topological feature. It’s important to know what feature you’d like to create beforehand, for example is it a point, line or polygon vector layer. When we create the layer, we must define what type of data it will contain. The purpose of this tutorial is to create a data type that can easily be manipulated, analysed and stored using a GIS system, hence the need to create vector files. We’ll create polygon features as an example;
Figure 7.5: New GeoPackage dialogue
At this point we must decide what kind of dataset we want to create. Remember that a data layer can only contain features of points, lines or polygons - never a mix. When we create the layer, we must define what type of data it will contain.
Since polygons are made up of points and lines, let’s create polygons. Once you’ve mastered this, creating a point or a line layer should be easy!
Within the dialogue, specify a file name for the new file, file encoding, geometry type, the CRS and add specify data for the New Field. Add other field names. This requires a predesigned data model that properly captures all information about the said feature.
Figure 7.6.1: QGIS canvas after adding ESRI World Imagery layer
Figure 7.6.2: ESRI World Imagery layer zoomed in
Figure 7.7: Digitizing tools
From left to right on the image above, they are:
We want to add a new feature.
Figure 7.8.1: Digitizing new feature
Figure 7.8.2: Adding the attributes
Figure 7.8.3: New feature created
If you make a mistake while digitising a feature, you can always edit it later. Simply finish digitising the feature and then follow these steps:
Move feature(s) tools | Move the entire feature(s) | |
Node tools | move only one point where you may have misclicked | |
Delete selected | get rid of the feature entirely so you can try again | |
Go to Edit ‣ Undo or press Ctrl+Z on keyboard | Undo mistakes |
To georeference the map;
Figure 7.9: Map in Georeferencer canvas
Next you should define the transformation settings for georeferencing the map:
Figure 7.10: Transformation parameters
When selecting the transformation parameters, the things to consider are:
Order of Transformation | Minimum GCPs Required |
---|---|
1 | 3 |
2 | 6 |
3 | 10 |
4 | 15 |
5 | 21 |
6 | 28 |
7 | 36 |
Just to be safe, always have at least one more than the minimum to add redundancy.
Figure 7.11.1: Enter map coordinates
Figure 7.11.2: Enter map coordinates
The first coordinate for the georeferencing is now ready. Below is a screenshot of what to expect at this point
Figure 7.12: First coordinate for the georeferencing
Zoom out in the image and move to the right until you find other crosshair, and estimate how many kilometres you have moved. Try to get ground control points as far from each other as possible. Digitize at least three more ground control points in the same way you did the first one. Tip: Make sure the points are fairly equally distributed across the image for example in all four corners of the image or at equal distances to each other. This affects the performance of the transformation algorithm. Which in turn results in higher error rates.
With already three digitized ground control points you will be able to see the georeferencing error as a red line coming out of the points. The error in pixels can be seen also in the GCP table in the dX[pixels] and dY[pixels] columns. The residuals should not be higher than a threshold that you set, if it is you should review the points you have digitized and the coordinates you have entered to find what the problem is. You can use the image above as a guide.
Figure 7.13: GCPs added
Figure 7.14: Georeferenced map loaded in QGIS
Note: To check that your data is properly georeferenced you can open the topo map. Your map and this image should match quite well. Set the map transparency to 75% and compare it to the aerial image.
a. the process of converting geographic data either from a scanned image or digital image into vector data by tracing the features
b. describes the pure analog-to-digital conversion of existing data and documents
c. process by which coordinates from a map, image, or other sources of data are converted into a digital format in a GIS
d. refers to creating a digital representation of physical objects or attributes
a. GPS data points
b. Topographic maps
c. Satellite imagery
d. Graphs and Tables
a. Resolution of the data (Spatial, Temporal, Radiometric)
b. Lighting
c. Location of the feature
d. Type of feature
a. get accurate data sources
b. set data quality goals
c. review the data and re-edit or revert edits
a. allows capture comprehensive information about the feature
b. automates the process
c. captures digitizing errors
Topogaphic map from https://legacy.lib.utexas.edu/maps/ams/east_africa_500k/