The Applications of Georeferencing in GIS
How the use of Georeferencing Tools Creates Accurate Maps with Satellite and Remote Imagery.
Introduction
Have you ever wondered why there are so many unique-looking maps? Most Americans are familiar with the Mercator Projection from primary and secondary school, but many would be surprised to learn that the shape of continents is highly distorted. The actual shapes and ratios may be dramatically different when viewed from space. Distortion is unavoidable when projecting a three-dimensional sphere onto a two-dimensional map.
On the right, several different projections serve a unique purpose that was considered by the cartographer(s) who designed them. Now how would we take a satellite or remotely taken image and correctly place them onto the globe? One answer is Georeferencing! This is done on a much larger scale map compared to the small-scale projections you see on the right. We can take many images, overlay them onto the map, and snap them into the proper geodetic location to create one seamless georeferenced photo. Remember that since the Earth is round, photos cannot be simply pasted onto the map. You may realize that some georeferenced images become extremely warped after you place several control points. Georeferencing is one of the many advances in technology to reduce the amount of distortion on maps we create. Before, we took an aerial photo and used that as a map, but now we can take multiple and create a digitally viewable map that has accurate coordinates.
Image obtained from Research Gate . https://www.researchgate.net/figure/The-nine-small-scale-map-projections-used-in-the-paired-comparison-test-arranged-by_fig1_273517879
Define Georeference.
The United States Geological Survey defines it as "...internal coordinate system of a digital map or aerial photo can be related to a ground system of geographic coordinates. A georeferenced digital map or image has been tied to a known Earth coordinate system, so users can determine where every point on the map or aerial photo is located on the Earth's surface" ( Link to USGS ).
Area of Focus: Downtown San Buenaventura, California
Selected Parcel Map Locations
Area of Focus: Downtown San Buenaventura, California
The City of San Buenaventura in Southern California is the seat of Ventura County. The parcel map images used to showcase the georeferencing process were all obtained from the Ventura County GIS website. This particular county still provides handwritten parcel plat maps to the public. These maps are not the most accurate since there is little geodetic information, but the geoprocessing tool in ArcGIS Pro allows us to manipulate the images into a more accurate representation of where the parcels would be located. This can assist us in creating a completely digital parcel map which is also available through the Ventura County GIS webpage. It makes the process of finding the correct parcel number much easier because you have the entirety of Downtown Ventura georeferenced; this helps us with our spatial awareness, which makes referencing much easier.
How Georeferencing Images onto a Map with Control Points Creates a Less Distorted Image
Although the images may look distorted while adding control points, they are actually more true to the way the surface of the Earth looks. One caveat to the geoprocessing procedure is that it is not the most accurate. What it can do is give estimated coordinates of where or parcel (or another feature from a different project) lies on the globe. It also makes life simpler for people who need to constantly reference a great number of images in a small amount of time. Searching on a digital map for something is much easier and more predictable than searching through a stack of papers.
