Defining coordinate reference system? [duplicate]

Question

In ArcGIS and QGIS you can add data on your map and it aligns perfectly even when the Coordinate Reference System (CRS) is not set. Both softwares inform the user that the CRS is not set and it should be done.

How the added data can align itself perfectly even if the CRS is not set?

After setting the correct CRS the data stays in exactly the same position and seemingly nothing changes. For me it seems that the CRS is originally correctly set in some unknown way.

Why does the user have to set the CRS manually again in both mentioned softwares, even the software knows the correct CRS already?

Answer 1

Alex has pretty much covered it (as mkennedy did in a comment) and I've upvoted his answer, but I want to offer slightly different wording/take on the issue.

All data you can add to a map has coordinate values. However it doesn't necessarily have what those values represent defined. Is it feet or meters or degrees? What is the origin? These kinds of things are covered with the CRS definition, and you can change the definition without changing the values. This is actually one of the most common mistakes made in GIS - defining coordinates as something they aren't rather than re/un-projecting them to something else and actually changing the values. See Layers with same coordinate system should align/overlap but do not?

If not defined, the software does one of two things - displays those coordinate values using the current map/dataframe's CRS; or displays them using a default coordinate system, such as geographic WGS84. In a way, you're right. The data was already using the 'correct' coordinate system, it just wasn't labeled as such. If the data's coordinates hadn't matched the system in use, it wouldn't have aligned correctly. The software doesn't know the correct CRS, it's just coincidence. By setting or defining the CRS you give the software the other piece of information it's missing, what those coordinate values mean, and it can then automatically relate them to coordinate values in other systems.

It's important to note that just because it appears to align doesn't mean it actually does. Some coordinate systems have very subtle differences, perhaps only a few meters or even less. Never assume things are aligned unless you know what the CRS is supposed to be, and if it simply appears to be right keep the fact you're making an assumption in mind - particularly depending on your accuracy/precision tolerances and requirements.

Answer 2

To put it simply, both software products do not ASSUME anything, as this creates a risk of working with data in two different projections and creating incorrect results without being aware of it.

In short, answers to your questions are as follows:

1. Spatial files may or may not have a coordinate reference system (CRS) set. If it is not set, the file still has coordinates, which must be in a CRS. The danger if not knowing what CRS your coordinates are in is that you might assume they're in one, when they're in another, and then you might be comparing apples to oranges and getting bananas as the answer.
2. You don't need to set the CRS manually if the software knows it. In QGIS, right click the layer in the legend and choose 'save as'. In this dialog, you can set the CRS. If it currently has one, the data will be transformed into the new projection (so the values in its X and Y coordinates will be changed). If it doesn't have one, it should be defined. In ArcGIS there is a function called 'define projection' (from memory), which will not alter data, but will write the CRS to the file.

So, to summarise. Your data itself has coordinates in a CRS. Your spatial file might have metadata that defines its CRS. And your DesktopGIS can override, or create this metadata and define a CRS. When you save data, you should try to do so including the definition of a CRS, as it helps others in the future (including future you).