What is the difference between datum transformation and map projection?

What is the difference between datum transformation and map projection?

A datum is simply a foundation and reference for spatial measurements. A system of coordinates is then used to describe those measurements relative to the datum, and a projection is the visual representation of those measurements on a different surface.

Is nad a geographic coordinate system?

The WGS 1984 and NAD 1983 coordinate systems are both earth-centered. When originally published in 1986, NAD 1983 and WGS 1984 could be considered coincident. Generally, coordinates in WGS 1984 and NAD 1983 are around one to two meters apart. GPS data is actually reported in the WGS 1984 coordinate system.

Why do I need to select the geographic transformation?

Project the data to the new PCS using the new GCS. You can see that when you select the PCS you want to use, you also need to select the geographic transformation (step 3 above) because there are multiple mathematical calculations that can be used to define how the coordinates will be converted to the new GCS.

When to use a transformation in projection on the fly?

Transformations are one part of the process used by the Project tool and projection on the fly. If you are converting between different geographic coordinate systems (GCS), you need to use a transformation. In the example above, the map is in a projected coordinate system (PCS).

How to select the correct geographic ( datum )?

It is necessary to specify a geographic (datum) transformation when using the ArcToolbox Project Wizard (ArcGIS version 8.x) or Project tool (ArcGIS versions 9.x, 10.x) to project shapefiles or geodatabase feature classes between different geographic coordinate systems (datums).

How to apply a geographic transformation in ArcMap?

Geographic transformations use different methods, with different accuracies, and are defined for particular areas. This document discusses how to manually apply a geographic transformation to the ArcMap data frame. In ArcMap, go to View > Data Frame Properties dialog. Select the Coordinate System tab.