The answer is these bearings are referenced to true north as indicated in the official AIP for Turkey:
However I don't understand why this reference to true north is not explicitly mentioned in the question chart, explanations are welcome.
The general recommendation from ICAO related to runway information is provided in Annex 4 to the Chicago convention:
CHAPTER 13. AERODROME/HELIPORT CHART — ICAO
13.6 Aerodrome/heliport data
13.6.1 This chart shall show:
d) all runways including those under construction with designation number, length and width to the nearest metre, bearing strength, displaced thresholds, stopways, clearways, runway directions to the nearest degree magnetic, type of surface and runway markings;
Annexes are only recommendations to States members, however when a State doesn't follow the recommendations it must provide a description of the deviations in the section GEN 1.7 of the AIP. In this case the deviations related to §13.6.1 are:
13.6.1.g TWY markings are not shown
13.6.1.i TWY centerline points and aircraft stands are not shown
No deviation for 13.6.1.d, so the runway bearings should be referenced to magnetic north. This is not a problem when reading documents where the reference is explicite (e.g. in the table above), but the aerodrome chart in your question has no warning.
A last point about bearings.
Before modern navigation instruments, in particular before GNSS, the compass was the reference. Today the magnetic reference creates many problems linked to how magnetic bearings are determined and used:
First a GNSS (or INS, or sensor-fusion) bearing is determined, it is true by nature as the coordinates reference system used for position determination has an ellipsoid aligned with Earth rotation axis (WGS-84).
Then tables are used to convert it to a magnetic equivalent because magnetic bearing is the legal reference for navigation, for crews and controllers.
While avionics silently uses the true bearing which is more simple, for all its calculation, all bearings displayed to the crew, or entered by the crew, will be magnetic.
The conversion tables are derived from the World Magnetic Model. All avionics units use this model, but different units may use different versions, and updates may be slow or inexistent. In addition units may use different algorithms and get results not matching or not matching with maps/charts.
So there is a project to stop using the magnetic reference and switch to a legal true north. Canada is driving this project for ICAO. Canada is indeed interested more than other because in polar regions, the magnetic declination varies a lot with position and also in time. So for these regions, and only for them, the legal reference is true north. Canada, among other, must live with two legal systems and make them more or less compatible.
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