I have a couple of maps on which I need to project a list of addresses. The problem is that the two maps are in "The Ordnance Survey National Grid reference system" i.e. "British National Grid".

If I project the long/lat of the addresses, obviously, they are not projected in the right spot. I tried a lot of things but I was not able to solve the problem.

Any suggestion?

I'm open either on transforming the long/lat in something else or transforming the maps.

EDIT: Let's say that I have one map and this map is a shapefile. I wrote that they are not correct because if I have a portion of map of one city (for example London) and I project the long/lat of an address located in London, in ArcGIS the point will be displayed far from the map. In order to project it I used the standard procedure of "display x-y data".

EDIT2: Sorry for the problems, but I'm not really an expert of Gis. I don't know if it is possible to put the URL of the map here, but basically I'm using the Land Registry's INSPIRE Index Polygons

EDIT3: the original file is in the GML format. I first tranform it in a shapefile then, when I import it in ArcGis, the program automatically recognize it with the Projected Coordinate System: OSGB_1936_British_National_Grid

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    Welcome to GIS SE. As a new user please take the Tour. Unfortunately, the meaning of "couple of maps" is unclear. Are they images? Why are lon/lat values "obviously" not projected correctly? What have you attempted to project them? What exact things have you tried, and how, exactly did they not work? Please edit the question to clarify these things. – Vince Oct 10 '16 at 12:53
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    The problem here is the meaning of "map" -- generally it refers to a finished cartographic product. A map can't be a shapefile, which is a data storage format, many of which are usually needed to make a map. There are two basic methods utilized for data projection -- One is used when you have data in a known projection and wish to copy it to a new data file in a different projection, and the other is to correctly define a data file with an unknown coordinate system, so that that ArcMap can project "on the fly" into the coordinate system defined by the map canvas. Which have you used? – Vince Oct 11 '16 at 3:35