I want to convert maps created in ArcMap (i.e. .mxd files) to a webmap. What is the best way to do that?
You could follow publishing map document on ArcGIS Online Publish hosted feature layers—ArcGIS Online Help | ArcGIS .
Then you can create a web map Make your first map—ArcGIS Online Help | ArcGIS followed by a web application Create apps from maps—ArcGIS Online Help | ArcGIS .
The workflow is similar in Portal for ArcGIS.
Retrieving data ...