At any given location on Earth, there are 3 ‘Norths’: True, Grid and Magnetic.

Magnetic can be ignored for the purposes of this discussion (suffice to say, it’s different from the other two).

True (solar/geographical) North

• TRUE (geographical) North is the direction of the geographical North pole, and relates to the rotational axis of the earth.

• True North is important for solar design, because the path of the sun across the sky is a function of the rotational axis of the Earth.

• Therefore PVsyst (and most PV calculation equations) assumes that ‘North’ in a model is True North.

• True North is an element in a ‘geographical’ coordinate system, described by angles (latitude/longitude) and elevation (above ellipsoid).

• Lines of longitude point at True North.

Grid North

• Nearly 100% of surveyors and cartographers draw maps in map projections.

• Map projections are a tool of convenience for humanity: they flatten a bit of the earth so it can be represented on a flat map.

• They also allow architects to ignore the curvature of the earth (because it’s not relevant at small scale) and represent 3D objects in Cartesian (XYZ) coordinate systems, relative to the surface.

• GRID North is only ‘north’ within the specific map projection that has been chosen.

• For example, the most common map projection for the UK is British National Grid OSGB36 (EPSG::27700).

• There are hundreds of different Grid Norths (one for each map projection), but only one True North.

• 3D location within a map projection is Cartesian, and is described by Eastings, Northings and elevation (above geoid).

• HOWEVER, lines of Northing cannot be consistently parallel with lines of longitude (unless you design a very exotic map projection).

• Therefore, lines of Northing do not point at True North (except at the grid meridian).

• All typical map projections, including UTM, have this limitation.

• Therefore, depending on where you are in the grid, then there will be a convergence angle between Grid North and True North.

• To the west of the grid meridian, True North will point due east (clockwise) of Grid North.

• To the east of the grid meridian, True North will point due west (anticlockwise) of Grid North.

• E.g. for the UK, the only place where Grid North = True North is at the -2° meridian (~Birmingham).

white lines = OSGB36 grid Eastings/Northings; red dots = lines of latitude/longitude (point at True North)

Why is this a problem?

• AutoCAD works in a Cartesian coordinate frame (basically a map projection). Surveys are typically done in map projections, and surveyors may not understand the importance of True North to a solar farm design.

• Many surveyors don’t mark True North. In fact, they cause confusion by drawing a compass rose, but pointing it at Grid North!

• We have observed that solar farms are therefore often designed in map projections, to Grid North. This appears to be an industry-wide issue, and unfortunately affects the majority of solar farm designs.

• Unless one rotates the map projection to match the local grid convergence prior to design stage, then the Y-axis will not align with True (solar) north.

• The further you are from the meridian, the bigger the difference (e.g. East coast of UK is particularly problematic).

• This has 2 problems:

• 1. Many solar farms that are meant to face True (solar) South, don’t actually face True South – they may be a few degrees off.

• 2. PVsyst (and other equivalents) assumes that North is True North, so many yield simulations contain this error.

What’s the solution?

• Ensure convergence angle is adjusted for prior to array design in AutoCAD. This means rotating the CAD elements around a fixed point, or using a user coordinate system (UCS) in AutoCAD, before using CAD-based design software e.g. PVCase.

• Also use these ‘true north’ versions for yield simulations, e.g. in PVsyst.

• But be aware that this rotation produces a non-standard CRS that will potentially be confusing for external sharing.

• Therefore, it is sensible/critical to counter-rotate models back into a standard map projection (e.g. OSGB36) for sharing externally (e.g. with constructors), since their surveyors will (generally) be expecting standard map projection data. This counter-rotation must be done around the same exact point as the original rotation.

• Above’s topographical CAD deliverables are typically delivered in local grid coordinates, as is convention for CAD, but we always mark the direction of True North with annotation of grid convergence angle, and note advice on rotation prior to design.

• OR, use a GIS-based (rather than CAD-based) system for design, but this may not be convenient or possible. Above can provide GIS deliverables where required.

Notes:

• ALWAYS ensure that coordinate systems (and any deviation from EPSG standards) are labelled very prominently on all drawings.
  
• EPSG::3857 (Web Pseudomercator) is the coordinate system used for nearly all online maps (including Google Maps). Whilst it’s not suitable for engineering drawings (it has scale issues at long distances), it does have the beautiful advantage that Grid North is aligned with True North. This is the coordinate system that all of Above’s online maps use for display. Therefore, ‘straight down’ on Above’s online maps is equivalent to True South.
  

Further reading