Coordinate systems
Coordinate systems help us establish location and direction, in other words they are the frames of reference for us to answer the questions, "where?". There are several types of dominant coordinate systems used on maps and globes and depending on which is used different information will be communicated.
Geographic Coordinate systems are based on angular measurements of latitude and longitude. These can be used with 3D globes (ellipsoids or spheres) or on a flat map projection. The location is expressed in terms of degrees.
Projected Coordinate systems use terms like "eastings" and "northings" to describe location. These coordinates are expressed in non-linear unites that are based on an XY axis. Projected coordinate systems use the information within geographic systems and link them to algorithms which define projection parameters. In other words, geographic systems exist within projected systems as a part of its foundational make-up.
Vertical Coordinate systems (VCS) define height and depth of features as they relate to mean low water (bathymetric) or mean sea sea level (topographic). VCS are defined within either geometric models (ellipsoid) or gravity related models (geoid).
Coordinate system information can be found by searching a Well Known ID or Well Known Text as defined by various authorities. Using the "define projection" tool within the Projections and Transformations section, metadata can be updated to provide the most up to date version of data.
Map Projections
Map Projections convert 3D imagery created with the data above into a flat 2D representation. Different projections exist because, depending on the information needed to be shown, a variety of distortions in shape, area and direction will occur when the 2D image is created. A projection intended to convey information about the North Pole will need a different projection than one intended to convey information about Hawaii for example. This is important for someone like myself, who lives and works in Alaska, to keep in mind.
Re-projections vs. Transformations
Re-projecting a map uses the same datum, the projection is converted to the Geographic coordinate system (GCS) and then re-projected back into the new desired projection.
Transformation is when the datum changes as well to create the new projection. So, the projection is converted to its GCS and then the GCS itself is changed for further projections.
Transformations may be necessary when base maps and data layers are aligned, on is possibly outdated or uses a different coordinate system. One example is local data mismatching with global data. This can create errors in visualizing and representing space and a transformation is needed to correct the issue by changing the datum. ESRI supports both datum transformations and vertical transformations. Pathways between data shown in transformation processes and the multiple transformations that may exist for one area can be found in the Project toolbar under "geographic transformation". These transformations will require Arc GIS Pro.
The define project tools can perform a variety of functions including helping to search for coordinate systems, filter information and save frequently worked with systems under favorites. You are also able to add a new coordinate system that is not already available, for example from a custom file provided by a client.
Creating a New Coordinate system
You can create a new coordinate system in Arc GIS pro for systems that are not defined by Esri or made custom for a certain purpose. Adding a new coordinate system under the new coordinate system tool within the coordinate system toolbox allows you to crate a new system with chosen values or import a system. These can be customized with different units, projections and scale factors.
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