I�??m really stuck with this and tearing my hair out!!! I wish to upload shapefiles which are in the OSGB 1936 co-ordinate system into Arc Explorer Online. These are shapefiles provided by Natural England: http://www.gis.naturalengland.org.uk/pubs/gis/GIS_register.asp
When the file is uploaded the polygons in the shapefile are shifted / misaligned by approximately 100 metres west and 25 metres south compared to the basemap. I have tried several transformations to other co-ordinate systems in ArcMap, to no avail. This included transformation to the WGS 1984 Web Mercator [Auxiliary Sphere] projection which is recommended here: http://help.arcgis.com/en/arcgisexplorer/help/index.html#//01560000003p000000
What am I doing wrong?! Please help! If I can get this to work, this ArcExplorer Online could be really beneficial to UK reptile and amphibian species.
If you have ArcMap, I'd recommend creating a new map and add the Topographic basemap (File, Add Data, Add Basemap), and make sure the Coordinate System of the Data Frame is set to the same coordinate system as the basemap layer (WGS_1984_Web_Mercator_Auxiliary_Sphere, WKID: 3857 in ArcMap 10.1). Add your data there, and try experimenting with the Transformations of the Data Frame until you are happy that the data is aligned as best it can be.
Then export the features to a new layer that uses the Web Mercator Aux Sphere coordinate syste, making sure you use that transformation - see the ArcGIS Desktop help topic Exporting Features. Basically by doing this, you can control the datum transformation yourself, instead of leaving it to the default used by the importing process in ArcGIS Online.
We generally transform data using the OSGB_1936_To_WGS_1984_Petroleum datum shift otherwise we get a ~100m data shift. The default projection transformation from OSGB36 -> WGS84 in ArcGIS has about 100m offset across the UK.
Try the workflow in the blog post and let us know if it doesn't work.
