I have an ArcMap basemap of Britain which has a coordinate system which is National Grid. I am trying to import a spreadsheet of coordinates of sites across Britain which are in decimal degrees (eg 50.933392 -1.5033064). However when I use the convert to x and y tool my data points are all projected into the sea off Cornwall. (Presumably my coordinate system is wrong so it puts the point in metres from a 0, 0 point. How do I sort this out?
Best bet. Bring them in with the coordinate system you know that they are first. Then convert them to the system you want.
Your initial points seem to be WGS84 or a geographic system. Bring them in using that coordinate system.
Then use the PROJECT tool to put them into the system you want.