by Victor Valquez
(Denver, Colorado, USA)
A certain amount of garden planning can be done on site without the need to prepare a detailed garden plan. However, when you are considering a large area or complete garden, the only way to plan successfully is on paper, in which case it is vital to have accurate information before commencing any planning work.
Accuracy at this stage will save enormous problems later, there is nothing more frustrating than having arrived at an entirely satisfactory detailed garden plan and to find on setting it out on the ground that it does not quite fit.
Sometimes even the smallest inaccuracy will mean compromise throughout the proposed design or, in the extreme case, necessitate a complete rethink of your plan. The object is therefore to record all the principal features in the garden plot as accurately as possible.
Surveying is a profession in its own right and there are limits to theaccuracy required for the average garden. However, if you have a very large garden with a lot of different features to measure, particularly if there are many level changes, it may be worth considering employing a qualified surveyor. The electronic equipment now available to the surveyor does mean that a large survey can be done more quickly and hence work out less expensive than one might expect.
For most gardens there are some basic techniques which can be employed, sufficient to establish the main features such as boundaries and their line. Invariably garden walls and fences are not square or parallel, even though they may seem so to the naked eye. Looking through the house deeds will often provide an OrdnanceSurvey of the plot at 1:2500 or larger, or your local library’s reference section.
This will assist a great deal. You can check the accuracy of the plan and its scale by measuring a few dimensions on site; even if you are unable to use the map as the measurements are not sufficiently accurate, you may well find that the angle of the boundaries will nevertheless be helpful and save some time carrying out a lot of cross measurements.
If you have an architect’s drawing of the ground floor plan of the house, this will also provide detailed information and save time. But beware, check some of the measurements as the builder may not have followed every detail on the drawings.
Before making any measurements it is a good practice to draw a rough diagram of the main features on paper as you see them, and indicate with arrows the dimensions you require to measure, on to this you can then add the figures when measured. This tends to avoid missing any measurements. The simplest method of undertaking thesurvey is to base all your measurements from the house walls. House walls, apart from being solid and permanent features, are usually square and can be used as a datum point.
In the event that none of the garden boundaries are straight, a line may be extended from a house wall across the area to be surveyed. This can be achieved by sighting to the line of the house wall and using garden canes (for surveyors’ ranging rods) to establish the line. (If it is not possible to sight a line from the end of the house, a line at 90′ from the house wall can be made using the 3, 4, 5 triangle method).
This is now a base line from which measurements can be made from any point along it. These can either be as offsets at 90′ or by using triangulation, in this way you can plot individual features such as trees. Triangulation may also be done from die house corners as these are fixed points, providing of course that a measurement has been made between them.
If you have been able to establish accurately the perimeter of the site, plotting the internal measurements is relatively simple. Deciding what is, and what is not important to measure is perhaps a more difficult task. Major items such as trees, retaining walls and paths should certainly be included. If in doubt at this stage it is better to mark them in as they can always be disregarded at a later stage.
Now is the moment to record on the rough diagram important notes on prevailing wind direction, sheltered spots, desirable views and the orientation and level changes. Without special equipment accurately establishing the levels of your site will be difficult. In many gardens however, the level changes may not be very great in which case a little guess work (or preferably an experienced eye) and at least an appreciation of where the changes in slope occur should suffice. However, one should bear in mind that it is often very deceptive and usually one tends to underestimate the extent of the changes in height.
Recording the height of existing retaining walls and steps can often give sufficient information. If you only require a few further levels it may be possible to do this using a long plank and spirit level and measuring the height difference.
If your garden does slope appreciably then it will be advisable to hire a level tripod and staff. You would then need to carry out the following procedure:
1. Accurately locate the position of the points where you wish to record the level heights. You may wish to make a grid in order to get a complete cover of the area, but you will still need to know certain specific positions such as the base of a tree which you intend to retain (you should not change the soil level around trees).
2. Decide on a permanent datum point (or bench-mark) such as the top of a manhole or area of paving that you know you are going to retain.
3. Set up the staff in a position that can be seen from all the points you wish to record, preferably centrally between them, and set the tripod level.
4. With your assistant holding the staff at each point, record all the levels you need to know. If not all the positions you wish to record are visible you will need to set up a new position. In this case having moved, refer back to the datum to assess the position of the new location to the previous recordings.
5. By adding or subtracting the figure recorded from the original datum, you will have the height difference from the datum to the desired level point.