The Draw widget in the Web AppBuilder for ArcGIS returns inaccurate measurements when the web map's basemap is an ArcGIS for Server map service, which was not originally projected to WGS 1984 Web Mercator (Auxiliary Sphere).
• Project the original data to WGS 1984 Web Mercator (Auxiliary Sphere) before publishing the map service to ArcGIS for Server.• Use the Measurement widget in the Web AppBuilder for ArcGIS instead of the Draw widget.