Practical 5 Basic GIS geoprocessing - Helminths 5 Basic GIS geoprocessing Modern Tools for NTDs...

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Practical 5

Basic GIS geoprocessing

Modern Tools for NTDs Control Programmes July 2017

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Aim of practical

This fifth practical aims to familiarize you with useful basic queries and techniques

in QGIS to extract information based on attribute data and using map insets.

Key learning skills

In this practical, you will learn to:

• Geoprocessing tools applicable to vector layers for extracting data using the

intersection function.

• Conduct basic queries.

• Use map composer to include a map inset to show the extent of the study

area.

This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported

License. This means that users are free to copy and share this material with others. Requests for

creating new derivatives should be sent to Jorge Cano Ortega ([email protected]).



Practical 5

1. Selecting data by using queries

Selecting data through queries in QGIS is a useful and quick approach for selecting

features in a vector layer. In previous sessions, you learned how to carry out this

basic operation by using a cursor to click on the desired feature.

Here, you will learn how to select data by querying its attribute values.

1.1. Select features by expression

In practical 2, we generated layers showing prevalence (by 100,000 inhabitants) for

lymphoedema and hydrocele. Display the layer with prevalence for lymphoedema:

Suppose you are interested in districts with morbidity rates for lymphoedema at, or

greater than 30.0 (per 100,000 inhabitants). Carry out this operation by:

• Right-click on CMR_LF_District_prevalence and open the ATTRIBUTE TABLE.

• Click on the ‘select features by expression’ icon to open-up a dialogue menu.



• In the centre panel click on FIELDS AND VALUES to expand the section. It will

provide a list of all the attributes available in CMR_LF_District_prevalence.

• Double-click on the field ‘PrevLYM’ to bring it into the left panel.

• Click on OPERATORS to expand the section. It will display a selection

ofmathematical signs for writing queries. Click on “=>” to bring it to the left

panel.

• To show districts with morbidity rates at, or greater than 30.0, the expression in

the left panel should be written as follows: “PrevLYM” >= 30.

• Click the SELECT button to complete the operation.



Districts with prevalence rates of lymphoedema at, or greater than 30.0 have been

selected – 12 districts have been highlighted in yellow. You can also view the

selected features in the attributed tables, these records will typically be highlighted

in blue.

Save as CMR_HighPrevLYM.shp – we will use this layer for geoprocessing in the next

section.

Type the number ‘30’

beside the expression



Note: Try using compounded queries to make more complex expressions in selecting

certain features. The image on the right shows adjacent regions (i.e. Sud-Ouest, Sud

and Littoral) that have boundaries faced towards the Gulf of Guinea.

The expression for this is:

"NAME_1" = 'Littoral' OR "NAME_1" = 'Sud' OR "NAME_1" = 'Sud-Ouest'

You can experiment with different conditions.

2. Geoprocessing tools applicable to Vector layers

Geoprocessing functions can be accessed from the QGIS toolbar by clicking on the

following tabs: VECTOR � GEOPROCESSING TOOLS. We will cover how to use

intersection tool for extracting features.

Layer needed for this section:

� CMR_Community_LF_morbidity.shp

� CMR_adm1.shp

� CMR_water_lines.shp

� CMR_HighPrevLYM.shp



2.1. Intersect

Intersection is a useful function for extracting data. To extract sites that are located

within (or intersects with) districts that have morbidity rates of lymphoedema above

30.0:

• Go to the VECTOR � GEOPROCESSING TOOLS and select Intersection. When

prompted, select a layer as an INPUT and INTERSECT. In ‘Input layer’ add the

layer of interest (CMR_Community_LF_morbidity).

• In ‘Intersect layer’ add the intersecting layer (CMR_HighPrevLYM). Name the new

layer as “CMR_LF_Sites_Intersect_Districts”. Click the RUN button.



Repeat the same process and extract the water lines and areas for the districts.

Name the output as “CMR_WaterLines_Intersect_District.shp”.

Your new output should look like:



2.2. Inset map frame

An inset map (or insets) is a smaller map, which is featured on the same page as the

parent map. Insets are typically displayed at a much higher scale showing a broader

view of the study area. This feature is very useful as readers are able to perceive

the size and extent of the area under study.

The image in the bottom-right corner is an example of an inset. It provides an overall

view of Africa and marks out the location for Liberia and Ghana allowing the reader

to perceive the whereabouts of these countries in Africa.

Use the map composer to create a map inset. Make sure the following layers are

active to produce the MAIN MAP:

� CMR_HighPrevLYM

� CMR_LF_sites_Intersect_Districts

� CMR_WaterLines_Intersect_Districts

� CMR_adm2 (display as grey background)

� CMR_adm1 (display as transparent with black outline)



Open map composer by clicking: Project � New Print Composer, and give it any

name you wish. We are going to add an inset at the bottom-right corner - create the

map in map composer as the MAIN MAP (refer to Practical 3).



You can change the layout of map canvas from landscape to portrait.

• Click on ‘Map 0’ in the item section to make it active.

• Click on the Composition tab, and select ‘Portrait’, in the drop-down menu under

the ‘Orientation’.

• Remember to LOCK everything.

Steps for adding a map inset to bottom-right corner:

• Create a duplicate from CMR_HighPrevLYM by right-clicking on it and selecting

‘duplicate’. This should add ‘CMR_HighPrevLYM copy’

• Rename this layer as CMR_MapInset_only, and change the fill colour to black.

• Only have the following layers displayed in the window:

� CMR_MapInset_only

� CMR_adm1 (transparent background, and black outline)

� CMR_adm2 (grey background, and black outline)

• Go back to the map composer and add a new map in the bottom-right corner by

clicking on and expand the cursor to make the map visible. Inset will be

appear as ‘Map 1’.



To make the study region in the main map reflected in the map inset:

• In the Item Properties for ‘Map 1’, click on Overviews to expand the menu.

• Click on the plus ‘+’ sign, this will add ‘Overview 1’. Under Map frame, select

‘Map 0’ to link the main map to the inset

map at the bottom.

• A red boarder will appear within the

insert inset frame.

• Lock the ‘Map 1’ to complete your map,

and export your map as an image.

The final output should look like:

Practical 5 Basic GIS geoprocessing - Helminths 5 Basic GIS geoprocessing Modern Tools for NTDs...

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