Identifying & Defining Coord System

[AmeliaNewsam]

I'm trying to make a map package, and the error comes up "Layers dataset is using an unknown coordinate system."  When I right-click and select "help" it takes me to an ESRI page (error 00047) where it first tells me I must 

• Define the coordinate system of the layer's dataset.

and then goes into detail on how to change the map's coordinate system, which is not what I need to do (the map is WGS 1984 UTM zone 10N).  When I go into the source data in the layer properties, it does not indicate any way to change the coordinate system there.  How can I do this?  How can I know that it is the correct coordinate system?  If I can't find the correct coordinate system, is that the end of my project?  

Thanks,

Amy

[DanPatterson_Retired]

in the toolbox... Define Projection—Data Management toolbox | ArcGIS for Desktop 

[AmeliaNewsam]

Thanks Dan - as usual, I came upon this the moment I posted the question.

Trying it out now. Not sure what happens though if I define it as the

wrong coordinate system. Guess I'll find out.

[DanPatterson_Retired]

yes... make sure you define it as what it IS and NOT what you want it to be 

A visual guide

/blogs/dan_patterson/2016/09/07/define-projection-vs-project-a-visual-guide 

[AmeliaNewsam]

thanks.

[MervynLotter]

If you are not sure what the projection of your shapefile/raster should be,

I find it is easier to include other shapefiles with a known projection (or

an ESRI basemap) within your map view, and then change the projection of

your map view until your unknown shapefile matches up perfectly with your

background data, then bingo, you have your projection to use in the Define

projection tool as Dan suggested.

[AmeliaNewsam]

That's a great idea, thanks Mervyn!