I've tried in a few locations to drop items, yet friends are not able to see them to pick them up. Do they take time to appear? Do users need to reload data to see these items?
I don't know true rate of map refresh, but in my inner feeling it's about 20-30 seconds or a bit more. Otherwise, it will be much more energy eating app.
Most efficient way to make items exchange: one, who will pick items will close application, other will drop all items he want and then only first player login, receiving latest map update and pick dropped items.
The refresh rate for your environment in Ingress is 30 seconds, as of January 2014. The client does it automatically; normally, no refresh is necessary.
Refresh time is a parameter that is set by the server when you log in, so Niantic could easily change it.
In my experience, the most-common cause for missing items is another player who has snatched them up; otherwise the cause might be a marginal Internet connection. You can try force-sync, but (esp. if your inventory is somewhat full) the sync might fail, too.
The easiest way to quickly see new items lying around is to hack a portal (assuming there is one of course). That triggers an environment update as a side effect, even if the portal is burned out.
By any chance were you driving just before dropping these items?
Ingress imposes a speed limit on movement; as near as I can tell, it's around 40 mph/65 kph. When you perform any action in Ingress, it's as if there is a circle drawn around that point which grows outward at about 40 mph. If you try to do something outside that circle (by driving to a new location too quickly) Ingress doesn't believe your location. Hacking fails. Bursters miss their target. And dropped items? They just seem to disappear.
I suspect burster energy and disappearing items actually appear on a line drawn from your last location to the current one at the 40 mph circle radius point, but have never been able to prove this by backtracking.
If your agent has been flagged for suspected TOS violation (e.g. gps spoofing) you might get about 90% dropped items destroyed by the server. To test this out, try dropping 20 items one at a time (e.g. 20 resonator) then you will see only 10% of those on the ground. If you are flagged then only about 10% will be visible. If you are speed capped then all dropped items should eventually return to your inventory.
Whenever you open and close a portal view, either by clicking on a portal or by key from inventory, the map gets refreshed. It's the fastest way to see changes on map such as dropped items.
- If you were flagged as driving due to high speed, you won't be able to drop items. (Dropped items and fired bursters will return to your inventory. Hacks will result in no items acquired.)
- DO NOT drop items if your location is inacurate, they will still get dropped but away from your real location!
This method is much faster and data saving than Force Sync or Logout-Login. It refreshes whole map - portals destroyed/deployed, links/fields, not only items dropped.
"Mastering*" this technique actually provides you significant advantage over normal players when fighting over portals.
*There isn't actually anything to master, other than simply know about this "feature" and when it is good to use.