Sunlight Studies

1. INTRODUCTION

The district plan makes provision for the protection of sunlight to adjoining properties and public spaces by way of Policies and Objectives. The associated Rules specify the permitted activity standards and assessment criteria give guidance as to what matters are of importance when making an assessment of effects. Shading is a function of direct sunlight and obstructions which prevent that sunlight from reaching its receiving environment. It is possible to predict the path of the sun through the sky in relation to objects which cause shading. Those results can then be displayed as tables of numbers or as graphs and charts.

District Plan building controls are typically defined by three static dimensions i.e. : geographical position (x,y) and height above datum (z). However, in the case of a sunlight study, it becomes necessary to add a fourth dynamic dimension, that of time (t).

In order to depict a sunlight study by way of plans or diagrams, it is necessary to make a series of calculations which inter-relate static positions such as buildings and hills within the receiving environment with the dynamic time component of the moving sun.

This gives rise to the two principal methods of analysis often referred to asl the Shadow Definition Method which freezes the time component and assess the size and length of shadows versus the Sun Transit Method which freezes a position and assesses shading as a function of time and date. Other names for a sun transit diagram include a solar chart or sun path diagram.

2. SOME TERMINOLOGY

To assist in understanding how sunlight is assessed it may be helpful to introduce some of the terms involved :

Latitude : an angular distance measured in degrees along the earths surface north or south of the equator

Longitude : an angular distance measured in degrees about the earth’s axis east or west from the primary meridian (Greenwich)

Azimuth : an angular direction between 0° and 360° measured clockwise from true north (as opposed to magnetic north)

Altitude : an angle of elevation measured up or down from the horizontal

Equinox : date when day and night are of equal length

Solstice : Winter solstice – around 22 June, shortest day & longest night

Summer solstice – around 22 December, longest day & shortest night

Transit : path across the sky

3. THE SUN’S PASSAGE THROUGH THE SKY

The earth axis is tilted by about 23.5° with respect to the plane of its orbit. It also wobbles slightly on its axis as it orbits around the sun.

This tilting means that the sun stays low in the sky during winter but passes high overhead in the summer. The height of the sun in the sky is also related to the latitude of where you are positioned on the earth. Wellington lies at a latitude of 41° south of the equator.

For Wellington at either equinox, the noon day sun will be at an altitude of 49°. At noon on a mid winters day the sun will only reach an altitude of around 25.5° (49 - 23.5) whereas in the summer it will climb to an altitude of around 72.5° (49 + 23.5).

A solar year starts on the summer solstice or longest day being the 22nd December, then runs through the autumn equinox, then the winter solstice or shortest day on the 22nd June, then the spring equinox before ending again on the 22nd of December.

The time of year also affects the direction at which the sun will cross the eastern horizon at sunrise and the western horizon at sunset. On 22nd December the sun rises in the southeast at an azimuth of approximately 120°, arcs high overhead in the northern quadrant reaching a maximum altitude of 72.5° then sets in the southwest at an azimuth of about 240°.

In mid-winter on 22nd June, the sun rises in the northeast at an azimuth of about 60°, arcs low overhead in the northern quadrant reaching a maximum altitude of around 25.5° before setting in the northwest at an azimuth of some 330°. The sun’s apparent height in the sky varies throughout the year between these extremes as the seasons change.

For any time of the day and for any day of the year, the sun’s position in the sky can again be defined by angles of azimuth and altitude which can be plotted out in the form of a sun diagram.

It is important to note that a particular sun transit diagram applies only to the area for which it was drawn as the calculations to show the sun’s path are a function of the observer’s latitude and the respective time zone. Separate diagrams can be drawn for each half of the year to take account of the suns varying rate of passage across the sky at different times of the year.

Using these diagrams, and with respect to a selected viewpoint, it is possible to determine at what times of day and on what days of the year, sunlight to the selected viewpoint may be affected. More on this later.

4. SUNLIGHT ACCESS PROVISIONS OF THE DISTRICT PLAN

District Plans typically include provisions for the protection of sunlight as a residential or public amenity. The most common regulatory controls are that of sunlight access planes applied to the boundaries of a site with respect to adjoining properties and maximum building height. The objective is to ensure that neighbouring properties continue to receive reasonable and appropriate levels of direct sunlight subsequent to any development work that may occur close by.

The Permitted Activity Standards allow for a certain level of shading to occur as a result of any proposed development work. However, the Rules require a Land Use Consent to be obtained where a proposal exceeds those Permitted Activity standards. A sun study is a useful tool for quantifying existing losses together with any future losses that may result from new development work.

With respect to loss of sunlight we must consider :

· How much sun is already received and at what times of the day (the existing environment),

· How much sunlight will be lost as a result of the permitted bulk of a new building,

· How much additional sunlight will be lost if the Rules are exceeded.

A full written report detailing the methodology used should accompany the sunlight study to assist with verification of the results. Durations of sun loss are often best summarised in tabular form. Whatever reporting format is chosen, it should clearly indicate the magnitude of losses that are expected to occur as a result of the development under consideration.

5. INFORMATION REQUIRED

Sunlight access controls are determined with respect to natural ground level along the site boundaries and maximum building height is measured from existing ground level over the site. A prerequisite to any sunlight study, particularly one showing regulatory horizon lines, is the preparation of a suitable site survey plan sufficient to accurately define :

· the position of the legal boundaries with respect to true north

· ground heights along boundaries (related to MSL where appropriate)

· the position and height of viewpoints relative to the boundaries

· the shape of the distant horizon

· the position and height of all existing structures which could affect sunlight, be they on the applicants property or adjoining properties

If maximum allowable building height is defined as a height above mean sea level, as for say the Oriental Bay Height Area, then the vertical datum for all measured heights including those shown on the plans must similarly be in terms of mean sea level.

6. SHADOW DEFINITION METHOD

The Shadow Definition Method works by freezing time and evaluating the geographical extent of shading to adjoining properties that would result from the proposed structure being built. Computer programs such as Autocad and ArchiCad utilise this method which basically depicts a birds eye view of the site showing the extent of resultant shadows that will be cast. We call this the “outside looking in” approach. Interpretation of results is often subjective and can be difficult depending on the clarity of presentation.

It is not proposed to elaborate on this particular method. In general terms, the study is undertaken by establishing the positions of existing foreground features such as houses and vegetation, then preparing either selected cross sections or a series of plan views that show the extent of shading for selected days of the year.

The calculations required to drape the extent of a shadow onto an irregularly sloping landform are quite tedious to do by hand. Computer software can be used but it is reliant upon having an accurate 3D model of the ground surface onto which the shadow can be draped. It is also difficult to incorporate background hills into such a computer model as these can influence the results.

Alternatively, selected cross sections can be drawn showing the inclined angle of the suns rays over various obstructions. Multiple lines can be drawn to show the effect for different times of the day on various days of the year. It is necessary to include the shape of background hills on the cross sections as they will influence early morning and late afternoon sun. The correct altitude for the sun must be determined with relation to the azimuth of the cross section and the chosen time and day of the year.

The only way to adequately assess duration of loss using this method is to increase the frequency of sampling. However, doing so can simply result in so many diagrams that it becomes difficult to understand the results. This method does not readily allow the number of days per year nor the time of day when losses will occur to be quantified.

7. THE SUN TRANSIT METHOD

The Sun Transit Method works by freezing a sample point (hereinafter referred to as a viewpoint) and graphically showing the suns path across the sky for that point in relation to various horizon lines. The advantages of this method tend to outweigh the disadvantages which can often make it the preferred method for assessing loss of sunlight. This method depicts what a viewer would see when looking out from a point on a site towards the distant horizon, i.e. “a view from the inside looking out”.

Given that duration of loss is more often than not the most important consideration, the Sun Transit Method becomes the preferred choice for assessment.

A sun transit diagram, is much like a panoramic photograph onto which has been superimposed an angular reference grid, the arc of the sun across the sky on various days of the year and the extents of any existing and proposed structures. The key to this method is the angular reference grid which is defined in terms of azimuth, a direction in terms of true north, and altitude, an angle of elevation.

The Sun Transit method is point specific and involves defining both physicaland regulatory horizonlines with respect to a selected viewpointthen determining the effects on sunlight at that particular point in relation to obstructions that could cause shading. Those obstructions can be either natural or man made.

Obstructions that constitute a physical horizon and that could cause shading include :

· the background horizon or skyline

· significant nearby vegetation

· existing buildings

· proposed buildings

Examples of regulatory horizon lines are :

· the sunlight access controls along boundaries

· maximum allowable height of buildings above existing ground level

· in some cases, existing use rights

Selection of Viewpoints

Given the point specific nature of this method, the selection of appropriate viewpoints becomes important. The best approach is to choose locations that are representative of a particular site such as just inside the window to an important room or in the centre of an outdoor courtyard. There is little point in selecting a spot that occupants seldom use.

Viewpoints are typically located on adjoining properties and a co-operative owner may offer advice as to a particular location that they consider that may be affected. If an adjoining owner chooses not to co-operate then it may be necessary to choose representative viewpoints regardless.

In any event, the adjoining owner should always be consulted as to areas of their property that are particularly important to them and for which any loss of sun would be detrimental.

Establishing Horizon lines

There are two principal ways of establishing the azimuth and altitude to a point for plotting on the sun diagram. The first is by direct observation using survey instruments and the second is by mathematical calculation.

7.1.1.1. The Existing Physical Horizon

The easiest way of establishing the physical horizon is to occupy the viewpoint with a theodolite, orient it to north and observe a series of azimuth and altitude angles to the various background and foreground features, be they hills, buildings or significant trees. Those observed directions can then be plotted directly onto the sun transit diagram and joined with lines to define the physical horizon. Alternatively, a compass and abney level could be used to determine directions in terms of magnetic north and altitude. However, it is necessary to increase observed magnetic bearings by approximately 23° to bring them in terms of true north. Failure to do so could result in significant errors which if discovered prior to say a hearing, could be used to discredit the reliability of evidence being presented.

If the selected viewpoint cannot be occupied for direct observation, then alternative survey techniques can be used to determine the 3D position of foreground features such as existing houses, so that azimuths and altitudes can be calculated for plotting on the sun transit diagram. It is possible to accomplish this without trespassing.

There are also other ways of approximating the distant horizon but they are beyond the scope of this presentation.

7.1.1.2. The Proposed Physical Horizon

Establishing the proposed physical horizon is slightly more challenging as it is based on mathematical calculations. Basically, it requires establishing the position and height of the proposed structure in relation to the viewpoint and calculating azimuths and altitudes using trigonometry so that the position of the structure can be plotted to the sun diagram.

Depending on the relationship of the viewpoint to the features being plotted, the resulting picture can appear quite distorted. For objects relatively close to the viewpoint, straight lines projected onto the angular reference grid should actually appear as curves.

7.1.1.3. The Regulatory Horizon

Establishing the regulatory horizon or permitted activity envelope is slightly more challenging.

The position and elevation of a number of points along the line of intersection of a plane representing the maximum building height for the site, and the sunlight access controls along the boundaries must first be determined. Knowing the corresponding position and height of the viewpoint, the azimuth and altitude between the viewpoint and the regulatory points can again be calculated using trigonometry for plotting on the sun transit diagram.

For anything but a flat site, this can be a challenging task. However, an accurate site plan allows this to be accomplished with reliability.

Interpretation Of Sun Transit Plans

For the selected viewpoint, a sun transit diagram should show :

· arcs across the plan to represent the suns path from east to west (from right to left) on indicated days of the year.

· “S” shaped lines generally at right angles to the suns path indicating the time of the day for the sun as it travels across the sky. To simplify matters, and given that the sunlight study is intended to assess duration of sunlight loss, times shown can be NZ Standard time. An allowance can be made for daylight saving if required.

· Horizon lines showing the extent of the existing horizon comprising both background and foreground features, the proposed horizon that will result from construction of the proposed structure and the permitted horizon which represents the extent to which a structure could be built without exceeding the Rules.

To read off a duration of sun loss, simply choose a day of the year and follow the suns path across the sky for that day noting the times at which each horizon line is crossed.

The duration of loss for that particular day is the difference between times. The longer the length of the suns arc below a horizon line, the greater the duration of loss. The period of loss can be estimated between the dates on which the loss starts and finishes.

Interpretation of the sun transit diagrams is greatly enhanced by drawing in reasonable detail, the buildings, hills and other features that make up the physical horizon lines. In essence, the diagrams should resemble a picture rather than a drawing covered with a series of nondescript lines. Colour can also be used to improve clarity. We have in the past used composite sun transit diagrams that show the suns path and various horizon lines superimposed over digital photography however, these are much more time consuming to prepare.

If the proposed structure extends above the regulatory horizon line, then, excepting small incursions permitted by the Rules (gable ends etc.) the loss can be attributed to the non-complying portion of the structure.

If a full east to west sun transit diagram has been produced, then it is also possible to gauge the extent of loss with respect to the total amount of sun previously received. That is to say, it will be possible to determine if the adjoining affected areas will still receive a reasonable amount of direct sunlight after the structure is completed.

If an adjoining property only receives limited sunlight, then any loss, no matter how minor, can be significant.

8. WHY CHOOSE SPENCER HOLMES LTD?

Spencer Holmes Ltd have been involved in sun study analysis since 1997. In that time we have successfully used the techniques we have developed around the sun transit method for all manner of resource consent applications, notified hearings and Environment Court proceedings where we have provided evidence in the role of expert witness. We are well placed to provide specialist consultancy services with respect to sunlight studies and shading analysis.