by Maarten Lambrechts
Almost all mapmaking tools with a graphical user interface use map layers to manage the content of the map. The source of the layers can be very diverse: they can come from the local computer or from a remote source, they can contain raster sources or vector source layers. But every layer has its own settings and styling, can be enabled or disabled, and can be moved up and down in the stack of map layers. So the map layers are the building blocks of the map.
Layers are the map units on which the map styling is defined. A raster file containing elevation data can be configured to render the elevation as different colours or as hillshading. A vector file containing the course of rivers, the layer can be styled to show simple blue lines, or the width of the river segments on the map could have a width depending on the flow rate of the river section, if that data is available.
So the styling of a map is configured at the map layer level. But there is more to layers than just styling: not all map layers are created equal.
Base layers help the map user to orientate herself better: important roads, building outlines and administrative boundaries give context to the other map layers. So base layers are the canvas to which other layers are added.
Thematic layers show the topical information of the map. These contain things like points of interest (like the location of shops or tourist attractions), live traffic information, inundated zones in a city, statistical information (like population densities) or a ‚You are here‘ marker. These layers are more important than the base layers, as they contain the information for which the map was created in the first place.
A special kind of layer are the text label layers. They contain the names of the features of a map layer. So a label layer is connected to either a base layer or a thematic layer.
To compose a map, layers need to be stacked on top of each other. The most important layers should be visible the best, so these should go on top of the map layer stack. This means that thematic layers should go on top of the base layers: you don’t want features that are included in the map only for giving context to hide the most important elements on the map.
Label layers take a special place here. Even though they might be connected to a base layer (like the streets in a city), it makes no sense to put them low in the stack, because this will lead to cut off labels, which are not really helpful for readers and make the map messy.
When stacking layers, you should also take into account the type of features in each layer. Point features are small, so if they need to be visible on the map, it is probably a good idea to stack them on top of lines and polygons. In the same way lines should in most cases be stacked on top of polygons.
Finally, it might be wise to consider the way map features occur in reality. Features that occur at a higher elevation in reality (like airplane flight paths, clouds, …) should probably also go high in the layer stack, while a thematic layer showing underground features should be low in the stack (unless of course it is an important thematic layer).
Instead of completely covering and hiding underlying layers, layers can also be made to blend into each other by setting some level of transparency to the top layer. The easiest way to do so is to apply some transparency to a layer, so underlying layers shine through.
Layers can interact in many ways with each other when transparency is applied to a layer. These different ways of blending are called blend modes. Below are the most common ones.
Normal blend mode lets the underlying layer shine through the top layer. It is a very straightforward blend mode, but it has one drawback: layers will appear to be faded and colours will look washed out.
Multiply blend mode is a solution to this problem. With multiply blend mode the colours in both layers keep their full intensity. The resulting colours are always darker than the colours of both blended layers.
Screen blend mode is the reverse of multiply blend mode: the resulting colours are always brighter than the colours on the blended layers. This can be useful if you are blending layers on top of dark base layers.
Baking the layer cake
When making a map, ask yourself the following questions to define your map ingredients:
- What is it that I want to show with this map? The answer to this question determines the thematic layers of your map.
- What other kind of information is relevant to the topic? The answer to this question defines the less important map layers.
- What kind of information could help to give the map reader some context? The answer to this question defines the base layers.
Remember that adding a new layer to a map is expensive: every new layer takes away attention from the reader, interacts with the other layers and makes the map more busy. So before adding a new layer, think about whether or not the map really needs it. Try disabling some of the layers in your map to see if this leads to a better or worse result.
Just as in baking a cake, the ingredients of a map and the order in which you add them determines the quality of the output. So make sure to stack your layers correctly: labels on top, thematic layers below that and base layers at the bottom. And then you can add some layer blending as icing on the cake.