A close measure gets harder when the jack and the candidate bowl are on different levels. A straight rigid tape can give a misleading path or nudge the equipment.
First confirm both objects are live
A jack can remain live in the ditch, and a toucher can be live there too. Confirm their positions and any boundary markers before comparing distance. A non-toucher in the ditch generally does not stay live simply because it looks close.
Use the right tool
World Bowls law 23.5 says a flexible or string measure should be used wherever possible when measuring between a jack in the ditch and a bowl on the green, or the reverse. Follow the contour without changing the jack or bowl’s position.
Measure nearest point to nearest point
Law 23.4 defines the endpoints: the nearest point of the jack and the nearest point of the bowl. Do not measure from centers or from a convenient mark on the bank. Have one person read and another watch for movement.
If something moves
Stop immediately. The laws distinguish displacement by a player, marker or umpire and provide restoration procedures. Call the official when the original position is uncertain rather than silently replacing an object by eye.
Source: World Bowls Laws of the Sport, laws 18, 23 and 37–38. Check your national regulations and event conditions before a competitive game.
A flexible measure protects both accuracy and the original head.
For a shared view of close heads, try WGTS for iPhone or WGTS for Android. Use the official measuring procedure whenever your competition requires it.