VFR ON TOP ref AIM
I thought for VFR flight you have to maintain sight of the ground at all times. This is to allow for emergency landings because how do you know there isn't a mountain there when forced to break through the cloud. Plus for navigating how can you see where you are going or where you are, a GPS isn't VFR.
The following is taken from the AIM:
a. Pilot.
1. This clearance must be requested by the pilot on an IFR flight plan, and if approved, allows the pilot the choice (subject to any ATC restrictions) to select an altitude or flight level in lieu of an assigned altitude.
NOTE-
VFR-on-top is not permitted in certain airspace areas, such as Class A airspace, certain restricted areas, etc. Consequently, IFR flights operating VFR-on-top will avoid such airspace.
REFERENCE-
AIM, IFR Clearance VFR-on-top, Paragraph 4-4-8.
AIM, IFR Separation Standards, Paragraph 4-4-11.
AIM, Position Reporting, Paragraph 5-3-2.
AIM, Additional Reports, Paragraph 5-3-3.
2. By requesting a VFR-on-top clearance, the pilot assumes the sole responsibility to be vigilant so as to see and avoid other aircraft and to:
(a) Fly at the appropriate VFR altitude as prescribed in 14 CFR Section 91.159
(b) Comply with the VFR visibility and distance from clouds criteria in 14 CFR Section 91.155, Basic VFR weather minimums.
(c) Comply with instrument flight rules that are applicable to this flight; i.e., minimum IFR altitudes, position reporting, radio communications, course to be flown, adherence to ATC clearance, etc.
3. Should advise ATC prior to any altitude change to ensure the exchange of accurate traffic information.
b. Controller.
1. May clear an aircraft to maintain VFR-on-top if the pilot of an aircraft on an IFR flight plan requests the clearance.
2. Informs the pilot of an aircraft cleared to climb to VFR-on-top the reported height of the tops or that no top report is available; issues an alternate clearance if necessary; and once the aircraft reports reaching VFR-on-top, reclears the aircraft to maintain VFR-on-top.
3. Before issuing clearance, ascertain that the aircraft is not in or will not enter Class A airspace.