Should the coordinate reference system of the map and the projection of feature class match?

[AmitaGnana_Prakash]

1) Is it necessary that the coordinate reference system of the map and the projection of the shape file should be the same?

2) When  the coordinate reference system of the map and the projection of the feature class were the same, the measure tool for measuring area did not display the units in sq.km. But it displayed in sq.km.

When only the coordinate reference system of the map was changed and the feature class projection was kept unchanged, the measure tool was able to display the units in sq.km. What could be the possible reasons?

[DavidPike]

1.  Not necessary to have the same projection, but the impact depends on what you're doing, it can be trivial to severe.

2.  Any data in the map will be projected 'on the fly' to the dataframe's projection.  I think using the tool - an area measure can only be used in a projected/planimetric coordinate system and can't perform a geodesic areal calculation, suggesting your Map coordinate system is in something like Web Mercator or WGS84?

[AmitaGnana_Prakash]

Thank you for the reply. Yes, I used the WGS84. 

Could you please share an example of the possible impacts of using different projections for the map and the feature classes

[DavidPike]

Really depends on extent, how many projections are involved and what type, if you have other large rasters of certain projections etc.

If you're zoomed-in to a city and have a shapefile containing millions of features etc, I would rather have the projection set to that of the shapefile rather than a basemap.

If my basemap was WGS84 and I wanted to look at a country and zoom/pan about, but happened to have a few simple shapefiles also, It would probably be slower to use the shapefile's projection.

Mostly it comes down to processing (how long you wait for the stuff to load up on screen).  Large rasters will take a long time to project on-the-fly, but so will large vectors.

Although this advice is for viewing the data - when you start doing geoprocessing operations and analysis, you need to be acutely aware of coordinate systems etc.

[AmitaGnana_Prakash]

Thank you, that was really helpful.