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I am updating some layers to arcmap and receing a warning saying "unknown spatial reference". My understanding is that in this case arcmap will try to adopt the data frame coordinate system (in the present case defined by the first layer uploaded).

The layer seems to be where it is supposed to be, but can someone confirm my thoughts?

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    It's just warning you that it doesn't know, project on the fly will be unavailable as the coordinate system of your map is unknown and is just coordinates (numbers). It would be best to use Define Projection to allocate the correct spatial reference before adding to ArcMap just in case there's a special transformation required for other data resources.arcgis.com/en/help/main/10.2/index.html#//… – Michael Stimson May 9 '18 at 23:04
  • The coordinate system of my dataframe/map is not unknown. If I right click on "layers" I have the coordinate system I want. The problem is when I import a shapefile that does not have a coordinate system assigned to it. It appears the above message, but arcmap is still able to draw this shapefile. How does arcmap does this? If I understand your answer it will use the coordinates of this shapefile on the coordinate system defined to my dataframe? – user120256 May 10 '18 at 0:16
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    I was assuming this was the first layer you added, my bad. Define your spatial reference properly, it's worth the time. The reason why you see it correctly is because the numbers match up but you can't rely on that all the time.. get into the practice of correctly defining your spatial reference or you run a risk of needing the data later and not being able to decide what coordinate system it should be in and then potentially defining it wrong - which is really bad. – Michael Stimson May 10 '18 at 1:21
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    If your data is undefined and in something like UTM and your map is in the same UTM projection it will draw in the right place. But if the dateframe (map) is in something like WGS84 the data will not projection on the fly to WGS84 because it doesn't know it's UTM. Always define your data to what it is, not what you want it to be, that way the program can do the correct math to end up in the right location. Always define the dataframe to what you want, or the first layer in defines it, and if that layer is undefined you may have data not projected correctly. – Bill Chappell May 10 '18 at 12:13
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ArcMap (and ArcGIS Pro's map and scene) can change a data layer's coordinates in-memory or "on-the-fly."

A data layer that has no coordinate reference system / spatial reference cannot be projected on-the-fly. The layer's coordinates will just be displayed as they exist. No assumption is made on what the coordinate reference system might be.

If a data layer with no coordinate system is added to ArcMap as the first layer, the data frame itself will have no coordinate system and you will find that the map scale control is grayed out, and some features of the measure tool don't work, etc.

As several people commented, if the data layer with no coordinate reference system does line up with other data layers, it probably has the same coordinate reference system as the data frame's. Where this can get tricky is that the data could be in a similar geographic coordinate reference system (datum) but not exactly the same as the data frame's. For instance NAD 1983 (2011) versus any of the earlier. If you're not comparing against very accurate imagery (sub-meter) or very specific features that are common to both layers, you won't be able to determine beyond a doubt the exact geographic coordinate reference system.

If you're reasonably certain that the coordinate reference system of the 'unknown' data layer is the same as the data frame's, use the Define Projection tool or the data layer's property page in ArcCatalog to set the coordinate reference system.

Note: coordinate reference system is an ISO term; Esri uses coordinate system. ISO's coordinate system has a different meaning than how Esri is using it.

Disclosure: I work for Esri.

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