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Below are two screenshots of the same data, featuring plots of land considered for a photovoltaic array. Depending on the projection used, there is a difference in the orientation of features.

(Taken on QGIS version 3.28.10-Firenze, but the question is not dependent on a specific version).

First, under projection EPSG 7794, RDN2008 / Italy zone (E-N) -- grid lines are slanting "up" to the right:

Plots of land in Sicily, with slightly North eastern tilt - Projection EPSG 7794, RDN2008 / Italy zone (E-N)

And the second one, of the same data with a different projection: EPSG 32633, WGS 84 / UTM zone 33N) -- grid lines are slanting "down" to the right:

Plots of land in Sicily, with slightly South western tilt - Projection EPSG 32633 WGS 84 / UTM zone 33N

My question is twofold.

Why do projections alter the features' directions? I would like to better understand the theory behind this phenomenon.

As for photovoltaic arrays it is critical that the real physical North is considered. How can I securely estimate the real North direction?

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  • Probably by using a grid in 4326, or any PV-software.
    – Erik
    Commented Nov 7, 2023 at 12:03
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    A little research into projections and how they transform a spheroid into a plane will go a long way (you'll want to focus on the difference between map north and true north). There are, of course, entire books on this topic, which is one of the markers of a Question which should be closed -- Quoting the Help Center : If you can imagine an entire book that answers your question, you’re asking too much.
    – Vince
    Commented Nov 7, 2023 at 12:09
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    Guys, please do not vote to close this. The question can be answered in few, simple words to a sufficient level. And as you can see, there are two answers already, both not as long as a book. If you think hard, any question on this site could be turned into a book but that really is not the point of this community Q&A system.
    – bugmenot123
    Commented Nov 7, 2023 at 21:04
  • WHat do you mean by "going up" and "going down"?
    – Martin F
    Commented Nov 7, 2023 at 21:28
  • I agree that this question should be reopened. OP asks a relatively straightforward question and provides relevant info (i.e., projections being used) with screenshots. The answers and discussion also help answer a very common question I get from users in my organization. And I think the Q&A can help others who are similarly confused about why orientations reported by GIS software differ when projections are changed.
    – lambertj
    Commented Nov 8, 2023 at 16:10

2 Answers 2

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As others have pointed out, orientations measured in GIS will always be relative to "grid north" for a particular projection. The relationship between grid north and true north varies depending on the projection, so it is best practice to be explicit when you report orientation values (e.g., 5 deg in NAD83 UTM 15N).

The relationship between grid north and true north also varies depending on your location, even within the same projection. For example, in UTM zones grid north is always "up" within each zone. Grid north equals true north at the center of the zone by definition, but as you move west or east away from the center of the zone the difference between the two increases. At the edges of UTM zones there can be a roughly +/- 2 degree difference between grid north and true north. Hopefully this diagram helps illustrate what I mean (red lines are true north, black dashed lines are grid north):

enter image description here

The difference between true north and grid north is known as grid convergence. There are methods to calculate it for a location. Some GIS software (e.g., ArcGIS Pro) also allows you to insert topographic north arrows that show the difference between grid north, true north, and magnetic north for a location shown in a map layout.

enter image description here

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The North of a projection refers to the projection's own definition.

Projections can be rotated or otherwise distorted in relation to the geometric body they project to a plane.

If you need geographical north, you need to determine it based on the geographical locations themselves.

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