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8/13/2019 Qgis.spatialthoughts.com-Tutorial Georeferencing Topo Sheets Topo Maps Satellite Image or Scanned Maps in QGIS

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http://qgis.spatialthoughts.com/2012/02/tutorial-geo referencing-topo-s heets.html?pfstyle=wp

Tutorial: Georeferencing Topo Sheets, Topo Maps, Satellite

Image or Scanned Maps in QGIS

Most GIS projects require georeferencing some raster data. In this tutorial I will discuss the concepts ,st rategies and too ls within QGIS to achieve a high accuracy georef erencing.

Georef erencing is the process o f assigning real-world coordinates t o each pixel of the raster. Many times

these coordinates are obtained by doing f ield surveys - co llecting coo rdinates with a GPS device for f ew

easily identif iable f eatures in the image or map. In some cases, where you are looking to digitize scanned

maps, you can obtain the coo rdinates f rom the markings on the map image itself . Using these sample

coordinates or GCPs ( Ground Control Points ), the image is warped and made to f it within the chosen

coordinate system. For advanced georeferencing using another georef erenced image or OpenStreetMap

data, see this tuto rial.

The data we will use f or this tuto rial comes f rom the excellent Hipkiss s Scanned Old Maps. There is a vast

collection of out -of -copyright scanned maps that one can use f or research. For this tutorial, we will use a

1870 map of southen Indiaand georef erence it using Quantum GIS. Download t his JPG image and save it

on yourhard-drive.

Georef erencing in QGIS is done via the Georeferencer plugin. If you do no t already have it, see this

tutorial f or f inding and loading plugins in QGIS.

Open the plugin f rom Plugins Georef erencer Georeferencer. A new plugin window like below will

pop-up. The plugin window is divided into 2 sections. The top section where the raster will be
http://qgis.spatialthoughts.com/2010/10/using-plugins-in-quantum-gis_05.htmlhttp://qgis.spatialthoughts.com/2010/10/using-plugins-in-quantum-gis_05.htmlhttp://www.hipkiss.org/data/maps/william-mackenzie_gallery-of-geography_1870_southern-india_3975_3071_600.jpghttp://www.hipkiss.org/data/maps.htmlhttp://qgis.spatialthoughts.com/2013/02/tutorial-advanced-georeferencing-in.htmlhttp://qgis.spatialthoughts.com/2012/02/tutorial-georeferencing-topo-sheets.html?pfstyle=wphttp://qgis.spatialthoughts.com/


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displayed and the bot to m sect ion where a table sho wing your GCPs will appear. If you wish, you may

drag the GCP window and separate it f rom the top section. Now click on the Open Raster button on

top-left corner.

Browse to the JPG image of the scanned map and click Open. In the next screen, you will asked to

choose the rasters coordinate reference system (CRS). This is the projection o f your control

points. If you have collected the data using a GPS device, you would have the WGS84 CRS. If you are

georeferencing a scanned map like this, you can obtain the CRS information f rom the map itself .

Loo king at our map image, the coordinates are in Lat/Long. There is no datum information given, so

we have to assume an appropriate one. Since it is India and the map is quite old, we can bet the

Everest 1830 datum would give us good results. Click OK.


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You will see the image will be loaded on the top section. You can use t he zoom/pan controls in the

to olbar to learn more about the map.

To assign coordinates, we need to use the map grid on the image. If you zo om to bot tom-lef t corner,

you will see markings f or 70 longitude and 5 latitude. So we can determine that the real-world

coordinate at the intersection o f these grid lines will be (70,5). Remember that X=longitude and

Y=latitude. Now click on Add Point button to add these coordinates. Once selected, click at the

intersection o f the 70 and 5 grid line.


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In the pop-up window, enter the coordinates. Click OK.

You will notice the GCP table now has a row with details of your f irst GCP.


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Similarly, take GCPs at all 4 corners of the image where you have grid intersect ions. The more GCPs

you have the better. Once you are done, click on Sett ings Transf ormation Sett ings.


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In the Transf ormation sett ings dialog, you will have to choose multiple parameters. To keep things

simple, choo se f ollowing parameters. We are set ting out Target SRS as WGS84, so the resulting

geot if f f ile will have a CRS of WGS84. Make sure you check the Load in QGIS when done button at

the bo tt om. Click OK.


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Now click the Start georef erencing butto n f rom the to olbar.

Once the process is complete, you will see the georef erenced geotif f image loaded in your QGIS

canvas.

The georef erencing is now complete. But as always, I encourage you to always verify your work. How

do we check if our georeferencing is accurate? In this case, load the country boundaries shapefile

from the Natural Earthdataset and compare them. You will not ice they match up pretty nicely. Thereis some error and it can be further imprived by taking more contro l points, changing transf ormation

parameters and trying a dif f erent datum.Hope you are able to learn this and achieve a good

georef erencing with your own data. Let me know in the comments how did it go.. Check out the

new tutorialAdvanced Georef erencing in QGIS using a Ref erence Layer
http://qgis.spatialthoughts.com/2013/02/tutorial-advanced-georeferencing-in.htmlhttp://naturalearthdata.com/

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