file conversion and geodatabase

8/19/2019 file conversion and geodatabase

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Exercise 3: File conversion and

geodatabase


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6 Downloading OpenStreetMap data and converting to geodatabase,

vector files and raster files

6.1 Introduction

Often the files that we use in a GIS need to be converted to other formats. We can think of the

following cases:

One raster format to another raster format: if we use a GIS for pre- or postprocessing of data

for a tool or a model, the data needs to be read or produced in the format supported by the

tool or the model. Thanks to the Geospatial Data Abstraction Library (GDAL) many

conversions are supported in commercial and open source GIS applications

One vector format to another vector format: similar to raster, but here often the OGR simple

features library is used to translate from one to another format.

From raster to vector: this is useful when analysis is done using vector geoprocessing

algorithms. This is also useful for presenting data to decision makers who often prefer data in

vector format. From vector to raster: in order to perform map algebra or other raster analysis, vector data

can be converted to raster data.

Conversion to and from geodatabases: instead of using files, GIS data can also be organized

in geodatabases.

In this exercise we will download OpenStreetMap data for your area of interest. OpenStreetMap

(OSM) is a collaborative project to create a free editable map of the world

(http://openstreetmap.org). Next we will import the data into a geodatabase. After doing some

queries, the resulting data will be saved in shapefiles. Some of the shapefiles will be converted to

KML to visualize in Google Earth and other shapefiles will be converted to raster in order to apply

map algebra.

6.2 Installing the OpenLayers Plugin

First we have to install the OpenLayers plugin. This plugin enables the visualization of web data, such

as OpenStreetMap, Bing and Google Maps. Follow these steps to install the plugin:

1.

In the main menu go to Plugins Manage and install plugins….

2. At Search type: OpenLayers.

3. Install the OpenLayers plugin

6.3 Installing the GeoCoding plugin

The GeoCoding plugin uses the Nominatim and Google web sevices to enable searching for an

address and get the coordinates (geocoding) or to click on the map canvas and get the address

(reverse geodcoding). This plugin will help us to easily find our area of interest. Now install the

GeoCoding plugin in a similar way as the OpenLayers plugin.

6.4

Search for your area of interest

Use the GeoCode plugin to search for your area of interest:

1.

In the main menu choose: Plugins GeoCode GeoCode

http://openstreetmap.org/http://openstreetmap.org/http://openstreetmap.org/http://openstreetmap.org/


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2. Type for example Kampala in the address dialogue.

3. If more than one place is found, you can use the dropdown menu to choose the right one:

4. A popup screen shows up where you have to indicate the CRS of the GeoCode layer (your

search will result in a layer added to the map canvas). You can choose EPSG:4326

6.5

Add OpenStreetMap to the map canvas

We can have a free map of the area of interest by loading OpenStreetMap to the map canvas. Do the

following:

1.

From the main menu choose Web OpenLayers plugin OpenStreetMap

OpenStreetMap.


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2.

Zoom in to your area of interest. You can move the GeoCode layer to the top, so

OpenStreetMap is in the background.

Now look at the map. What kind of features can you see? Is the map vector or raster?

6.6 Downloading OpenStreetMap vector data

The OpenStreetMap data loaded in the map canvas comes from a web map service. In fact the

original data is vector, but it has been rendered to a raster image to be presented in a fast a nice

way. Often, however, we do not only want to visualize the topographical map of the area of interest,

but also use the data for analysis. For this purpose we can download the OpenStreetMap data of our

area of interest, using the following steps:

1.

Zoom in to your area of interest (e.g. center of Kampala).

2.

In the main menu go to Vector OpenStreetMap Download Data…


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3. In the dialogue choose From map canvas for the extent and browse to select an output file

name (e.g. Kampala.osm). In case internet does not work, this file has been provided so

you can continue offline with the exercise.

4. Click okay and wait until the data has been downloaded. If the popup with the message

Download has been successful appears, click OK and Close to go back to return to the map

canvas. Note that sometimes with a poor internet connection the download can be

incomplete, but no error message is displayed. This results in an empty attribute table. If this

occurs, try to download the file again or use the file from the Open CourseWare website.

5. The file you have downloaded is a .osm file. This is an XML format that is supported by OGR.

Therefore, you can open it easily in QGIS. Click the Add vector layer button and browse

to the .osm file. Click Open.

6.

A popup window shows up and asks you to select one or more layers from the file. Click

Select All and OK .


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6.7 Querying vector data

Let’s assume that we are interested in the primary roads. We can make a selection from the vector

attribute table and export the selection to a new vector file.

1. Click right the lines layer (lines any) and choose Open Attribute Table…

2. Inspect the attribute table. You can see that it contains for example street names and road

types. Now we’re going to select the primary roads from the attribute table. Click (Select

features using an expression) from the toolbar on top of the attribute table.

3. In the dialogue that opens, we can build the expression for the selection of primary roads.

a. First click on the + sign before Fields and Values

b.

Double click on highway. You can see that “highway” has been inserted under

expression.

c.

On the lower right, under values, you can click all unique load all unique values.

d. Click on the = button

e.

Double click on ‘primary’. Now the expression should look like this:

f. Click Select and Close.


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4. In the title of the attribute table window you can see the number of selected features out of

the total. On the lower left you can click on Show selected features.

6.8 Exporting vector data

1.

In the map canvas these selected features are highlighted in yellow. We can now export the

selected lines to a new vector file in any OGR format (e.g. Google KML or ESRI Shapefile).

Click again right on the lines layer (lines any) and choose Save as…

2.

In the dialogue choose ESRI Shapefile as the output format. Save the file as

PrimaryRoads.shp. Here you can also change the CRS of the output file. Click on the

select CRS button, choose EPSG:21096 and click OK. This is the projection used in Uganda,

north of the equator (UTM Zone 36N/Arc 1960).


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3.

Click Save only selected features (if we don’t check the box we can convert the entire lines

layer to another OGR format), check the box to Add saved file to map and click OK .

4. Repeat the procedure to select from the points layer (points any) the manmade towers

(“man-made = ‘tower’) and save the results to Towers.shp


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5. Repeat the procedure to select from the polygon layer (multipolygons any) the

commercial buildings (“buildings” = ‘commercial’) and save the results to

Commercial.shp

6.9

Vector to raster

If we want to calculate for example Euclidean distance from commercial buildings, which is a rasteroperation, we need to convert our shapefile with commercial buildings to raster format.

1. First we need to add a column to the attribute table of Commercial.shp, so we can add a

value of 1 for the existing buildings. In the attribute table click on to toggle to editing

mode.

2. Next, click on to add a new column. In the Add column dialogue type Value at the Name

of the column. Choose Whole number and give 1 for the width and click OK .

3. Now we can type the expression directly at the top of the attribute table: Value = 1. Click

Update All . Now the Value column will be filled with ones.

4. Click to toggle back and save the edits.

5.

From the main menu choose Raster Conversion Rasterize (Vector to Raster)…


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6. Fill in the dialogue as below and click OK , OK , OK and Close. This results in a Boolean raster of

3000 x 3000 pixels with value 1 for the commercial areas.

Use the Identify tool to check the values of commercial and non-commercial pixels.

6.10 Saving the database

Instead of using separate .tif and .shp files, we can also save the layers in a geospatial database.

Here we will create SpatiaLite database, a file based database.

1. Go to the Browser and click right on SpatiaLite and select Create database

2. Browse to the folder where you want to save it and give it a name, e.g. Kampala.sqlite.

A popup window informs us that the database has been created.


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3. Now we have to fill the database. In the main menu choose: Database DB Manager DB

Manager .

4. A new window appears where we can control the databases in QGIS. Click on the + sign

before SpatiaLite to see the databases. Then select Kampala.sqlite.

5. The database is empty and we need to import the layers. Click on the Import layer/file

button .

6. Choose the layer to input. Start with the Tower points. Click Update options to pre-fill some

of the form option. Specify the source SRID and target SRID as 21096. Enable the check boxto create a spatial index. Click OK to perform the import.

7.

Click OK when the popup shows up that the import was successful.

8.

Click the refresh button to see the added layer in the database.

21096 21096


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9. Repeat the same steps for PrimaryRoads and Commercial. Note that we cannot import

raster data. Finally the DB Manager should have the points, lines and polygons.

Close the DB manager.

6.11 Adding layers from the database to the map canvas

1.

From the Layers list remove all vector layers, so we only have OpenStreetMap and the

CommercialRast. You can do this by click right on the layer name and choosing Remove.

2.

Click the Add SpatiaLite Layer button .

3.

In the dialogue that shows up make sure the right database is chosen, e.g.

Kampala.sqlite, and click Connect.

Select all the layers and click Add . The layers will be loaded into the map canvas.

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file conversion and geodatabase