Does anyone know what this symbol on aerodrome charts is?
It is the Aerodrome Reference Point, ARP.
As per ICAO annex 14 vol 1, chapter 2.2:
2.2 Aerodrome reference point
2.2.1 An aerodrome reference point shall be established for an aerodrome.
2.2.2 The aerodrome reference point shall be located near the initial or planned geometric centre of the aerodrome and shall normally remain where first established.
2.2.3 The position of the aerodrome reference point shall be measured and reported to the aeronautical information services authority in degrees, minutes and seconds.
The FAA likes to use the term "Airport Reference Point"
Please see the Jeppesen AWM introduction - airport chart legend (pdf). Chart on page two has the symbol with refrence number 17, and on page three this is explained as:
17 — The geographical location of the Airport Reference Point (ARP) is depicted when known.
It is a symbol for a referenced location as defined in the “Legend” that explains what each symbol means. That legend, which must accompany a map for it to be useful, is usually found within the first few pages of the directory of airports or at the very end of the book.
I’ve seen that symbol used on some non-government map providers as the Airport Reference Point (ARP) which is stated as the “Location” of the airport using geographical coordinates (N dd mm ss and E ddd mm ss) in the text notes about the airport.
ALWAYS refer to the legend to determine what a symbol means!
The circumscribed cross icon in SACAA charts appears to be a non-descript icon for a radionavigational beacon. The type is not indicated; could be VOR, TACAN or NDB.
They seem to be more common on "sectional" charts than for AFDs; the AFDs I've checked on the SACAA website show FAA/ICAO-style nav beacon types. Compare this Airspace Infringement Hotspot chart for the Cape Town Intl. area with the AFD for the same airport: the beacon's location, but not type, is marked in the wider map, while the more specific symbol for the VOR station is used on the AFD.
This is opposite FAA norms, where the sectional chart (intended to be used from the air to get into and out of airports) shows the specific beacon type with frequency info, while they're usually only shown on the AFD if they're somewhere in the airport that you can taxi to (making them an obstacle you should avoid while moving around the airport), and even then, they're typically displayed as a "black box" indicating a building. The beacon icon on FAA AFDs is usually only for rotating lights.