Coordinate systems allow geographic datasets to use common locations so that they can be viewed and used together.
This is a big topic in itself (click here to read more), but to get it started, a coordinate system is a reference system used to represent locations within a common geographic framework. There are two types of coordinate systems. One is a Geographic Coordinate System (GCS) which is global or spherical. The other is a Projected Coordinate System (PCS) which provides various mechanisms to project the earth's spherical surface onto a two-dimensional plane. A spatial reference is the georeferencing and coordinate system assigned to geographic data. It defines how geographic data is mathematically transformed onto a flat map. The correct spatial reference settings and transformation allow geographic data that has different coordinate systems to line up in a GIS.
Several coordinate systems are at play within the CT ECO website. We receive a lot of questions about this and what do to when datasets seemingly do not line up. Hopefully this helps.
The map and image services available on CT ECO are in WGS 1984 Web Mercator Auxiliary Sphere, which is the current standard for web services. In ArcMap, look for WGS_1984_Web_Mercator_Auxiliary_Sphere. EPSG 3857 (formerly 102100).
The downloadable 2016 tiles and mosaics are in the CT State Plane NAD 83 (2011) Feet. This is the most current realization of NAD 83 published by the National Geodetic Survey and is consistent with the national Continuously Operating Reference Stations (CORS) Network. In ArcGIS, look for NAD_1983_2011_Connecticut_ft. EPSG: 6433.
When using layers together that have different spatial references, it is CRITICAL to set the correct transformation. In ArcGIS, a Geographic Coordinate System Warning appears when the layer does not match the spatial reference of the data frame to which it is being added. Do not ignore this! Choose the correct transformation, such as the example below where the image service (in GCS WGS 84) is being added to a data frame that is in GCS NAD 83 (2011).