The Basis of Bearing and Bearing Datum can be one in the same, however, most of the time they have different meanings.

The Basis of Bearing as shown on a survey is defined between to physical monuments such as a Section corner and 1/4 corner, along a boundary line or road centerline. This gives an angular relationship for all of the bearings shown on the survey. Most governing agencies require a Basis of Bearing on a survey or plat for recordation. The absence of a Basis of Bearing does not invalidate a survey or legal description.  

Prior to GPS, conventional survey equipment was used to turn angles and measure distances between monumentation. A basis of Bearing was identified to lock the survey into a physical position on the ground that could be used for retracement purposes.  

The requirement for a Basis of Bearing has been around for a long time. Legal descriptions, surveys, ALTA's, etc usually make reference to a basis of Bearing to aid in giving a surveyor a starting point or frame of reference for the courses of a survey. With today's modern GPS equipment, the relevancy of a Basis of Bearing is of less importance. Because tradition is hard to change, the Basis of Bearing will more than likely always be required on surveys that will be reviewed by governing agencies and for land title companies. The reality is, that any course on a survey can be referred to as a Basis of Bearing to meet a checklist item requirement for submittal to a reviewing body. The reviewers don't care which line is used, they just need to complete their checklist. Maybe the Basis of Bearing requirement should be removed from a checklist and leave it up to the surveyor's discretion.  

Every course will be related to some Bearing Datum. The Bearing Datum is not usually stated on a survey or legal description. A Bearing Datum can be assumed, magnetic, astronomic (true) or Geodetic (grid). When a bearing datum is not stated on a survey or legal description, it is assumed that it is meant to be a True bearing. When surveying a parcel, the adjoiner parcel descriptions/plats will also need to been analyzed. Most of the time the bearings will be different along common boundary lines. This can be confusing for a non-surveyor. The common boundary lines, monumentation and angular relationships is what ties several parcels together and not the bearing datum.  

All surveys should be tied to controlling monumentation such as the Public Land Survey System, national survey system, local control networks and/or accessories to give a survey and/or legal description longevity and to make it easily retraceable .  

Retracement of a survey or legal description is the key. Enough information needs to be provided so that the survey and/or legal description mathematically closes and can be easily retraced by any surveyor. 

I cannot remember how many legal descriptions, surveys and plats that I have retraced that was missing vital information, especially for curved segments, that is needed to retrace the boundary. Local tangent bearings, radial bearings or chord bearings are often missing from legal descriptions, etc.  

The subject of curves is topic for another discussion at a later date.