I have 2 files added to my map of Oregon: an Oregon hillshade (.sid) layer, and a .shp file that consists of about 20,000 monitoring points. The hillshade layer was added first and therefore the map/data frame takes on the same projection as the hillshade layer (NAD 83 Lambert Conformal Conic). The Geographic coordinate system of the points is different (NAD 83 2011) from the GCS of the hillshade layer. So, in order to make the coordinate systems match, I go to the Define Projection tool and change the point layer to match the hillshade layer by selecting NAD 83 Lambert Conformal Conic from the list. The map then redraws and my points are now positioned to the left of the state and also scaled very small.
I've done a ton of reading on datums, projections, GCS vs. PCS, etc. but I don't understand what is happening here. Can anyone help?
Defining the projection does not actually change the projection. You need to define the projection as what the datas projection actually is and then use the project toolbox to actually project the data.
And to add to Robert's suggestion... the only time that you ever define a projection using the define projections tool
is when you add a file to a data frame and you get a warning that the coordinate system is not defined. You can check any layer by going to its properties and checking to make sure.
When you want a file to be in a particular coordinate system, you use the project tool
Project—Help | ArcGIS for Desktop for vector
Project Raster—Help | ArcGIS for Desktop for raster
Projecting produces a new file... that is the key ... it changes the coordinates into new ones. Which is why you don't define something as something it isn't. You/It need to know what the coordinates are. If they are unknown, define them. If you want them to be different, you make sure they are defined and have a coordinate system first... then you can make them into something different.