One common issue i come across during my training activities is how to ensure that vector layers created using different softwares align properly on same georeferenced raster image. A large cross section of my trainees use either the opensource QGIS and/or Arcmap Desktop softwares. A lot of times, they realize that the vector data created using this different software apps don’t align with each other when placed on the same map i.e. a layer created in QGIS will not align in the same position when it is placed on the same map in Arcmap.
We will demonstrate how to ensure this doesn’t happen, as it would serve no good purpose if your postgis database is full of tables that contain geographic data that don’t align with each other.
To get this right, we should ensure the following guidelines below are adhered to:
a) Coordinate Reference System for the vector layer and raster image should be the same b) Image or raster should be for georeferenced using one software and exported for use on others as a geotiff file.
As you will observe from our demonstrations, the above guidelines will be applied in order to achieve the desired results.
We will be using two popular desktop GIS apps- QGIS and ArcMap 10.4.1. to demonstrate. For this part one of the post, we will start with ArcMap 10x.
1. First we will georeference our raster image (Topographic Sheet 261 Ibadan NW) using ArcMap 10.4 . Launch Arcmap and right Click on layer and select ‘properties’ from the dropdown menu, We must set the CRS of the layer frame the raster image will seat in.
2. In the Data Frame Properties window, Click on the Coordinate System (A ), for this demo, we wil use the local datum for our image- [EPSG 4263] minna ( B) and click OK
3.Now let us add the raster image for georeferencing. Click on the Add button as shown on the screenshot below.
4. We pick the raster image- samplemap.tif
5. Click ‘yes’ to build pyramids. take the other default options.
6. This message box will show as the raster image has not been georeferenced earlier so lacks coordinates assigned to the pixels. Click Ok
7. Use the georeferencing toolbar to georeference the raster image.
Georeferencing raster images using Arcmap has been publlished on this blog earlier. Yoi can go to mapsnigeriainitiative.wordpress.com ,site and search for it.
8. Once the image is georeference, appropriate map coordinates will appear at the bottom right cornet of the map canvas. See red boxed area on the screen shot below.
9. Next we create a new shapefile. Go to the Catalog tab at the right hand side of the arcmap screen and click on it. The Catalog tree will open, connect to your working folder and right click – Select New- Shapefile
10. The Create New Shapefile dialog box will open. For the demo, we will pick polyline for under Feature Type and click on Edit tab (lower red boxed area) to select and input appropriate coordinate system. We title the shapefile – linefeature.
11. The shapefile will be created and automatically placed on the layer window. Click on it and go to the Editor toolbar and start an editing session. (Digitizing vector layers using Arcmap 10 has been covered in earlier posts on this blog).
12. We digitize a road feature on the raster image. See the line feature in red running across the topo map.
Remember we said one of the parameters that determined proper alignment of the vector data was to have a uniform source for the georeferenced raster image.
13. Therefore we now proceed to save our georeferenced image as a geotiff file so that it can be loaded as a layer into other GIS applications, as in our case, QGIS. Right Click on the raster layer- samplemap- select Data – Export Data from the dropdown list.
14. A dialog box opens A) Under Extent, we select using the Data Frame (current) which will cut out the section of the raster image that is shown on the map canvas. B) Select TIFF as format and input a name for the file. The file would by default be saved in the same file as the original raster image.
We can now access the georeferenced raster image as a TIFF file in QGIS.
15. Launch QGIS Desktop and go to the catalog tree. Click on your working folder and navigate to where the TIFF file is saved. Double click on it to place it on the map canvas.
16. Now we navigate to the shapefile-linefeature created in ArcMap and double click on shapefile to load it onto the map canvas. As you will observe on the screenhot below, the vector data aligns properly on the raster image as it did in arcmap.
Note that the shapefile was created in Arcmap. therefore we have been able to keep the proper alignment even in QGIS.
Now let us create a shapefile layer in QGIS and test the alignment on the same raster image in ArcMap.
17. Click on Layer – Create Layer – New Shapefile to create the shapefile.
18. In the New Shapefile Layer dialog box, we select a feature type that is a line. Make sure the project crs align with that of the newly created vector data.
19. We name the new shapefile-linefeature2 and save it in the working folder.
20. Click on the toggle edit pencil icon to start creating the feature. (digitizing layers in QGIS is covered in an earlier post).
21. A road feature on the GeoTIFF is digitized. See green line on the image in the screenshot below. Note that the image had been converted to GeoTIFF format and loaded into QGIS.
23. We navigate to where the shapefile is stored using on the catalog tree and add it onto the layer window. From the screenshot below, our shapefile- linefeature2 created in QGIS now fits snugly into the same image in ArcMap.
*Contrary to my usual practice of making available a sample data for our tutorials, we are constrained from sharing this map because of rights and privileges that the National Lands and Survey Office has over it.
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