Qfyilyi

Background:

I have both QGIS 3.4.1 and ArcGIS Desktop 10.6.1 installed on the same laptop, Windows 10.

I have a polygon shapefile that is projected to Lambert Conformal Conic. Arc states that the shapefile's EPSG value is 102004. Here's a screenshot from the Arc Layer Properties:

spatialreference.org agrees with Arc; 102004 is Lambert Conformal Conic:

So far, so good!

Problem:

QGIS states that this layer's EPSG value is different: 100027. Here's a QGIS screenshot:

But wait, there's more! spatialreference.org does not have an entry for 100027. Here's a screenshot:

Question:

Why doesn't the QGIS EPSG value match Arc and spatialreference.org? What does the 100027 value represent?

when you add a new custom projection to QGIS, it allocates a unique code to it and stores it in a local sqlite database (srs.db)

the first one you add is numbered 100000, the second is 100001... and numbers are allocated sequentially each time you do this. As @user30184 says, this is to prevent duplicates.

your 10027 doesn't correspond to the corresponding arcgis/spatialreference.org codes. It means that this is the 28th custom projection you've added (based on the uniqueness of the proj4 string for the projection)

The projection is shown as 'USER:10027', rather than 'ESRI:10027' or 'EPSG:10027', which suggests that it's not equivalent.