Georeferencing Raster Imagery in SAGA GIS - Revision history

Jonathanpierre: /* Creating Ground Control Points */

2018-10-05T20:40:19Z

Creating Ground Control Points

Jonathanpierre: /* Data Acquisition */

2018-10-05T20:31:54Z

Data Acquisition

Jonathanpierre: /* Data Acquisition */

2018-10-05T20:31:11Z

Data Acquisition

Jonathanpierre: /* Transformations */

2018-10-05T20:30:30Z

Transformations

Jonathanpierre: /* Defining Projection */

2018-10-05T20:30:14Z

Defining Projection

Jonathanpierre: /* Creating Ground Control Points */

2018-10-05T20:29:58Z

Creating Ground Control Points

Jonathanpierre: /* Creating Ground Control Points */

2018-10-05T20:29:29Z

Creating Ground Control Points

Jonathanpierre: /* Creating Ground Control Points */

2018-10-05T20:28:39Z

Creating Ground Control Points

Jonathanpierre: /* Defining Projection */

2018-10-05T20:26:58Z

Defining Projection

Jonathanpierre: /* Creating Ground Control Points */

2018-10-05T20:26:24Z

Creating Ground Control Points

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-In order to georeference an image, we use '''Ground Control Points''' (GCPs). these are points that establish a relationship between the pixel coordinate system of the raster to a coordinate system of the earth. GCPs are typically placed at sharp features such as intersections or easily visible landmarks, but for this map, we will use sections of the map graticule with known coordinates.
+In order to georeference an image, we use '''Ground Control Points''' (GCPs). These are points that establish a relationship between the pixel coordinate system of the raster to a coordinate system of the earth. The connection between these points is said to be a Link: the complexity of the image will define how many numbers of links are needed to rectify the image. To get the best results, one must georeference the image to the highest resolution. It will facilitate the process of shifting the raster datasets from its original location to spatially corrected location.  Also, Adding more control points can increase the overall accuracy of a geo-referenced image transformation. [http://desktop.arcgis.com/en/arcmap/10.3/manage-data/raster-and-images/fundamentals-for-georeferencing-a-raster-dataset.htm Source]  GCPs are typically placed at sharp features such as intersections or easily visible landmarks, but for this map, we will use sections of the map graticule with known coordinates.
-GCPs could be defined as a point on the surface of the earth of known location which is used to geo-reference image data sources, such as remotely sensed images or scanned maps.
-It will help in shifting the raster datasets from its original location to spatially corrected location.
-One control point from a raster dataset associates with one and only control point from the target map that is a geo-referenced raster or vector data.
-The connection between these points is said to be a Link: the complexity of the image will define how many numbers of links are needed to rectify the image.
-To get the best results, one must georeference the image to the highest resolution
-the more control points which are created, the more accurate the georeferencing will be (Arcgis, 2013; 2016).
 # To start creating the GCPs, we will run the '''Create Reference Points''' module. in the '''Tools''' tab, select '''Projection -> Georeferencing -> Create Reference Points [Interactive]''''.

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 #Open Google Earth, and zoom to the area of interest of your choice. For this tutorial, we used the area around the Smithsonian Museum of Natural History in Washington, DC, USA, but you may also pick the location of your choice.
 #In the '''Layers''' menu in the bottom left corner of your screen, uncheck all layers. Google Earth renders terrain in 3D, and the perspective this creates can distort your exported imagery (See Figure 8).<br/><br/>[[File:LAYERSSAGA.PNG|thumb|center|Figure 8: Layers Menu of Google Earth]]<br/><br/>
-#Ensure that your imagery is north-facing and vertically-angled by selecting '''View -> Reset -> Tilt and Compass''' in the menu bar at the top of the screen (See Figure 3)<br/><br/>[[File:TIltcompass.PNG|thumb|center|Figure 9: Resetting the Angle and Rotation of Google Earth]]<br/><br/>
+#Ensure that your imagery is north-facing and vertically-angled by selecting '''View -> Reset -> Tilt and Compass''' in the menu bar at the top of the screen (See Figure 9)<br/><br/>[[File:TIltcompass.PNG|thumb|center|Figure 9: Resetting the Angle and Rotation of Google Earth]]<br/><br/>
 #Under '''Tools -> Options''', ensure that under '''Show Lat/Long, Universal Transverse Mercator''' is selected.
 #Using the '''Add Placemark''' tool (See Figure 10), add four or more placemarks to your screen. make note of their coordinates and label them numerically. change their icon to one that is more precise, location-wise.<br/><br/>[[File:Placemarks.PNG|thumb|center|Figure 10: Adding Placemarks in Google Earth]]<br/><br/>

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 #Ensure that you have downloaded and installed [https://www.google.com/earth/ Google Earth] on your computer
 #Open Google Earth, and zoom to the area of interest of your choice. For this tutorial, we used the area around the Smithsonian Museum of Natural History in Washington, DC, USA, but you may also pick the location of your choice.
-#In the '''Layers''' menu in the bottom left corner of your screen, uncheck all layers. Google Earth renders terrain in 3D, and the perspective this creates can distort your exported imagery (See Figure 2).<br/><br/>[[File:LAYERSSAGA.PNG|thumb|center|Figure 8: Layers Menu of Google Earth]]<br/><br/>
+#In the '''Layers''' menu in the bottom left corner of your screen, uncheck all layers. Google Earth renders terrain in 3D, and the perspective this creates can distort your exported imagery (See Figure 8).<br/><br/>[[File:LAYERSSAGA.PNG|thumb|center|Figure 8: Layers Menu of Google Earth]]<br/><br/>
 #Ensure that your imagery is north-facing and vertically-angled by selecting '''View -> Reset -> Tilt and Compass''' in the menu bar at the top of the screen (See Figure 3)<br/><br/>[[File:TIltcompass.PNG|thumb|center|Figure 9: Resetting the Angle and Rotation of Google Earth]]<br/><br/>
 #Under '''Tools -> Options''', ensure that under '''Show Lat/Long, Universal Transverse Mercator''' is selected.

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 Because images obtained from aerial photography may be distorted, often it is necessary to apply a '''Transformation''' during the georeferencing process. The relevant coordinate transforms are typically stored within the image file (GeoPDF and GeoTIFF are examples), though there are many possible mechanisms for implementing georeferencing. Different transformations are available at  the following transformations are available in SAGA GIS.
-[[File:GUID-7A915DDE-DF6C-4FEF-AC1B-C1AED1A1BACB-web.gif|thumb|center|Figure 21: Transformations]]
+[[File:GUID-7A915DDE-DF6C-4FEF-AC1B-C1AED1A1BACB-web.gif|thumb|center|Figure 22: Transformations]]
 =====Triangulation=====

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 #Open the '''Set Coordinate Reference System''' module. in the '''Tools''' tab, select '''Projection -> Proj.4 -> Set Coordinate Reference System'''. Select the '''User Defined''' field, and in the popup menu, change the '''Projection Type''' field to '''Universal Transverse Mercator (UTM)'''. in the '''Predefined Datum''' Field, select '''North_American_Datum_1927'''. In the popup menu, press '''Okay'''. (See Figure 20) <br/><br/>[[File:Setcoord.PNG|thumb|center|Figure 20: Setting a Coordinate System]]<br/><br/>
-# In the module menu, under the '''Data Objects''' section, select the '''Grids''' Field. A popup menu will open up. Select your image file and click the '''>''' button to transfer it to the right side of the popup menu. Click '''Okay''' to finish. (see Figure 21) <br/><br/>[[File:Gridselect.PNG|thumb|center|Figure 20: Selecting a grid in the Data Objects Menu]]<br/><br/>
+# In the module menu, under the '''Data Objects''' section, select the '''Grids''' Field. A popup menu will open up. Select your image file and click the '''>''' button to transfer it to the right side of the popup menu. Click '''Okay''' to finish. (see Figure 21) <br/><br/>[[File:Gridselect.PNG|thumb|center|Figure 21: Selecting a grid in the Data Objects Menu]]<br/><br/>
 ====Georeferencing the Grid====

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 <br/><br/> [[File:Action_button.PNG|thumb|center|Figure 17: Action Button]]<br/><br/>
 # Repeat this process at least four more times, each with a different point. Once you have completed this, end the module by selecting '''Geoprocessing -> Create Reference Points [Interactive]''' in the menu bar at the top of the screen (See Figure 18). <br/><br/>[[File:Endmodule.PNG|thumb|center|Figure 18: Ending a Module]]<br/><br/>
-#After adding the GCPs to the image, you can have a look at the added GCPs and the pixel coordinates associated with them. '''Go to the data tab -> Right click on the Reference Point (Origin) layer  ->  Attributes -> Show''' <br/><br/> [[File:Sagagis attributetable.png|thumb|center|Figure 18: GCPs attribute table]] <br/><br/>
+#After adding the GCPs to the image, you can have a look at the added GCPs and the pixel coordinates associated with them. '''Go to the data tab -> Right click on the Reference Point (Origin) layer  ->  Attributes -> Show''' <br/><br/> [[File:Sagagis attributetable.png|thumb|center|Figure 19: GCPs attribute table]] <br/><br/>
 #If at any time you need to edit, save or delete any Control Points collected, this can be done by accessing the View Link Table button on the Georeferencing taskbar (highlighted in red below). This table displays the Control Point(s) information as well as the Transformation and Auto-Adjust options which control how the raster image responds to the addition of control points. Each line represents a Control Point, a link between the raster image and the digital map data. Both the Source and Map Points can be edited by double-clicking on the values in the table. Clicking on the Save button allows the points table to be saved as a text file while the Load button allows for a set of control points stored in a text file to be loaded and edited. Saving a text file of your control points is strongly recommended. For images requiring many control points to georeference, periodically saving your control points to a text file allows you to come back to your work in case you are not able to finish it in one session. <br/><br/>

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 According to our map, the projection used is '''NAD 1927 UTM'''.
-#Open the '''Set Coordinate Reference System''' module. in the '''Tools''' tab, select '''Projection -> Proj.4 -> Set Coordinate Reference System'''. Select the '''User Defined''' field, and in the popup menu, change the '''Projection Type''' field to '''Universal Transverse Mercator (UTM)'''. in the '''Predefined Datum''' Field, select '''North_American_Datum_1927'''. In the popup menu, press '''Okay'''. (See Figure 10 <br/><br/>[[File:Setcoord.PNG|thumb|center|Figure 19: Setting a Coordinate System]]<br/><br/>
+#Open the '''Set Coordinate Reference System''' module. in the '''Tools''' tab, select '''Projection -> Proj.4 -> Set Coordinate Reference System'''. Select the '''User Defined''' field, and in the popup menu, change the '''Projection Type''' field to '''Universal Transverse Mercator (UTM)'''. in the '''Predefined Datum''' Field, select '''North_American_Datum_1927'''. In the popup menu, press '''Okay'''. (See Figure 20) <br/><br/>[[File:Setcoord.PNG|thumb|center|Figure 20: Setting a Coordinate System]]<br/><br/>
-# In the module menu, under the '''Data Objects''' section, select the '''Grids''' Field. A popup menu will open up. Select your image file and click the '''>''' button to transfer it to the right side of the popup menu. Click '''Okay''' to finish. (see Figure 18) <br/><br/>[[File:Gridselect.PNG|thumb|center|Figure 20: Selecting a grid in the Data Objects Menu]]<br/><br/>
+# In the module menu, under the '''Data Objects''' section, select the '''Grids''' Field. A popup menu will open up. Select your image file and click the '''>''' button to transfer it to the right side of the popup menu. Click '''Okay''' to finish. (see Figure 21) <br/><br/>[[File:Gridselect.PNG|thumb|center|Figure 20: Selecting a grid in the Data Objects Menu]]<br/><br/>
 ====Georeferencing the Grid====