Base map and overlays

What a base map and overlays are for

Every design needs a sheet of paper that tells the truth about the ground. The base map is that sheet: a drawing to scale of everything on a site that stays where it is. Boundaries, buildings, large trees, paths, fences, wells, ponds, slopes. It carries a north arrow and a scale, and it leaves empty margins for notes. Nothing planned goes on it, and nothing that is only a guess.

What has been observed and what is being planned goes on overlays: transparent sheets laid over the base map, one topic per sheet. One shows where water runs and gathers after rain, one the sectors of sun, wind and fire, one the zones of daily use, one the soils, and the last the design itself. Each sheet can be read alone, redrawn alone, and laid together with any other. Where two layers meet, design questions appear that no single map would raise: a moist channel running through the area walked every day, a fire sector crossing the place meant for a woodpile.

Bill Mollison treated zones, sectors and slope as one piece of planning in Permaculture: A Designers' Manual (1988). The base map is where the three finally sit on the same page.

How to make one, step by step

1. Choose paper and scale. Measure the longest side of the site and pick a scale that fits it on the sheet with room to spare. For a garden, a scale such as 1:50 on a large sheet is common; for a farm, a smaller scale or several sheets.
2. Set a baseline. Run a tape along one straight edge, usually the longest boundary, and draw it lightly. North points up the page.
3. Measure fixed points. From each end of the baseline, measure to every corner, tree and building. On paper, draw an arc for each distance with a compass; where the two arcs cross is the point. This method, trilateration, needs nothing more than a tape, a compass and a scale ruler.
4. Or trace an image. An aerial or satellite image with a scale bar can be projected or printed at the chosen scale and traced. It is quicker, but corners and tree positions should still be checked on the ground.
5. Add the shape of the land. Contour lines show how water will move. They can come from a topographic map, from elevation data, or on a small site from an A-frame or a water level used to mark points of equal height.
6. Make copies. Before anything else, copy the base map several times at the same scale. Overlays get scribbled on, and the base must survive.
7. Draw one overlay per topic. Water, sectors, zones, soils and existing vegetation each get their own sheet and their own colour. Date each sheet; a summer layer and a winter layer of the same topic often differ.

From tracing paper to LiDAR

Overlays began as paper. The landscape planner Ian McHarg made them famous in Design with Nature (1969). To find the least harmful route for a new road, his team drew each factor on its own transparent map, darker where the cost to land or people was higher, and stacked them. The route showed itself where the stack stayed lightest. Teaching material from Penn State University describes this as the root of the map overlay that geographic information systems later turned into software.

Today the same logic runs on a computer. A free program such as QGIS stacks layers just as tracing paper does, and it can calculate new layers from old ones. The most useful data for land design come from airborne LiDAR. A plane sends laser pulses to the ground and records their return; according to NOAA, the result is a dense cloud of points with position and height, from which elevation models and contours are made. From an elevation model one can derive slope, aspect toward the sun and the lines along which water is likely to collect. In Italy, the national environment ministry and several regions, Sardinia among them, publish LiDAR terrain models for parts of the country, often along rivers and coasts.

Digital layers are precise, but they are not observation. They do not show where people really walk, where the soil is compacted, where a neighbour's dog crosses or where the wind breaks branches. Those overlays still come from boots on the ground, a notebook and a year of observing.

Common mistakes

• Plans on the base map. Once a planned bed is drawn on the base, nobody remembers what was really there. Keep the base clean.
• Everything on one sheet. A map carrying water, wind, zones and plants at once becomes unreadable, and nothing can be changed without redrawing the whole.
• Trusting the screen over the land. An elevation model does not know about the culvert under the track or the spring that appears only in March. Check every important line in the field.
• Missing scale or north arrow. Without them, a copy or a photograph of the map can no longer be measured, and overlays drift out of register.
• Mapping only once. Water, sun and use change through the year. Overlays drawn in one season need a second look in the opposite one.

Further in the TerraWiki

• Sectors and zones are the first two overlays almost every design draws.
• The design process shows where mapping sits between observation and design.
• Keyline design and the swale depend on accurate contour lines.
• Microclimate is easier to read once slope and aspect are mapped.
• Indicator plants and the spade test feed the soil and vegetation overlays with what the data cannot see.
• Design from patterns to details explains why the large layers come before the choice of plants.

Maps of Sa Medada

For Sa Medada, a south-facing slope near Jerzu in Ogliastra, there are maps of elevation, slope, wetness and solar aspect derived from LiDAR data. The slope rises from 247 to 341 metres, and its flanks reach gradients of 25 to 45 degrees. The wetness map shows a comparatively dry main area around the house and two seasonal watercourses along the sides of the land. The solar aspect map marks the warmest and most sheltered parts of the slope.

These maps are the base on which swales and keyline lines are to be laid out. Swales are planned; none has been built yet.