Ask any experienced deer hunter where to look first on a cold autumn morning, and the answer is usually some version of the same thing: find the sunny faces. It is received wisdom in the field, the kind of knowledge that gets handed down without much explanation. The explanation, it turns out, is almost entirely a GIS problem — and someone has now built a tool that makes it quantifiable.

Solar irradiance modelling and shadow analysis are established techniques in the geospatial toolkit. They are used in photovoltaic site assessment, urban planning, agriculture, and ecological modelling. What they have rarely been applied to, at least in a form accessible to ordinary users, is hunting. The HuntingNZ web map has changed that for hunters operating in New Zealand’s mountain country.

The Physics Behind the Pattern

Animal behaviour follows energy. On a cold morning, a deer or tahr that has spent the night bedded in a sheltered hollow needs to warm up. The fastest way to do that is to move to a slope that receives early direct sunlight. On a hot afternoon, the same animal needs to shed heat, so it moves to north-facing shade in the Southern Hemisphere, or to the cool air settling in gullies and valley floors.

This is not just folk knowledge. It is the intersection of thermoregulation biology and terrain geometry, and terrain geometry is something GIS is exceptionally good at describing.

The key variable is slope aspect — the compass direction a slope faces — combined with the angle of inclination and the position of the sun in the sky at any given time. A southeast-facing slope in mid-winter gets a very different solar budget from a northwest-facing slope on the same day. Add in the shadowing effect of adjacent ridges, and the pattern of sunlit and shaded terrain becomes complex enough that intuition alone starts to break down.

What Shadow Modelling Actually Calculates

In GIS terms, solar analysis over terrain involves calculating the solar zenith and azimuth angles for a specific geographic location, date, and time, then projecting the expected shadow cast by every elevated terrain feature across the surrounding landscape. The computation uses a digital elevation model to determine what blocks the sun’s path to any given point on the surface.

The output is a map of direct sunlight hours per pixel across the area of interest. It accounts not just for slope aspect and gradient, but for topographic shading — the shadows cast by ridgelines and peaks onto adjacent terrain. This is the part that matters most in hunting country, because ridgeline shadows can leave entire valley floors in shade for most of a winter’s day even when a slope fifty metres higher is bathed in full sun.

Running this calculation correctly is computationally demanding. It requires a high-resolution elevation model, accurate solar position algorithms, and integration over time — not just a single snapshot, but the accumulated solar radiation across the hours when the sun is above the horizon.

The Tool in Practice

The HuntingNZ map includes a sun and shade analysis tool that makes this accessible without requiring any GIS background. A user draws a box on the map — up to ten kilometres by ten kilometres — selects a date, and the tool calculates and displays how many hours of direct sunlight each part of that area receives, including the shadow effects of surrounding ridges.

From a GIS standpoint, the inclusion of topographic shading — not just aspect-based insolation — is the technically interesting part. Many simpler implementations approximate solar exposure from aspect alone, ignoring the cast shadows from adjacent terrain. Including ridge shadows produces a materially more accurate picture, especially in the kind of deeply dissected mountain topography that characterises the New Zealand high country.

The tool also allows users to step through the time of day, watching the shadow line move across the terrain as the sun tracks through the sky. This is useful for planning approach routes and timing a stalk to coincide with when a specific face is likely to hold animals.

A GIS Technique Finding a New Audience

Solar analysis tools exist across a wide range of professional GIS platforms. What makes the HuntingNZ implementation noteworthy from a geospatial perspective is not that the technique is new — it is that the application is so clearly matched to a specific, practical decision a specific group of users needs to make.

The question a hunter asks is: where will deer be at 8am on the 15th of June? The question a GIS analyst asks is: which areas receive direct solar radiation in the morning hours on the winter solstice? These are the same question, framed differently. The tool translates between them.

For anyone working in applied geospatial analysis, that translation work — embedding terrain analysis inside a domain-specific decision context — is increasingly where the interesting product development is happening. The sun and shade tool on HuntingNZ is a useful and surprisingly sophisticated example of it.