Map layout

 

Visualising data

Thematic maps that differentiate between features on the basis of their attribute information rely on colour and/or shading to emphasise the range of attribute values associated with the map features. Selection of colours and shades has an important bearing on the visual impact of the map, and how easily a user can interpret its message. Inappropriate use of colour and shading can ruin an otherwise excellent map! The following advice is based on our experience reviewing seabed habitat maps during the MESH Project. It is not intended to be a prescriptive or exhaustive list, rather just some simple ‘dos’ and ‘do nots’ to remember when deciding on a colour scheme to apply to a map of the seabed:
 
Remember to:
 
  • Take account of how the colours of adjacent polygons will appear when they are shown on the map: are they possible to distinguish at the ‘normal’ viewing scale?
  • If possible, check the appearance of colours on other computer screens or a variety of printers since devices often render the same colour slightly differently.
  • Reflect on the relative sizes of polygons in your map. For example, in a map that incorporates mapping of both the intertidal and subtidal zones, the subtidal polygons are usually larger than the intertidal polygons. Larger polygons are better illustrated with pale colours to prevent them to visually dominate the map. Use bold colours to highlight the smaller intertidal areas of the map.
  • Remember that a user will often view the map at different scales either broad scale (zoomed out) or fine scale (zoomed in). Electronic mapping systems can allow the user to zoom in and out freely and at very broad scales a colour scheme applied to a very detailed level of a seabed classification scheme might become impractical.
  • Consider users with colour vision deficiency (colour ‘blindness’), especially those who are unable to distinguish between red and green.
  • Investigate how an existing colour scheme will look if it is applied to your map. This may add value to the map, as users accustomed to the current colour schemes can easily interpret it. MESH has designed a colour scheme for EUNIS (2004 version), which can be used to symbolise polygons attributed with EUNIS habitat types. Below level 3 the EUNIS habitat types are grouped so that habitat types can only be distinguished at level 3 or above; for example, A1.1.
  • Think about how the majority of users will view the map – printed or on screen? If the main usage will be electronic in GIS packages, the colour scheme can be kept relatively simple because the standard query tools allow users to get more detail if required. If the map is to be printed, ensure that your chosen colour scheme will be adequate at the planned printing scale.
  • Record your colour scheme and store it together with the file. For example, you could create an ArcGIS™ style if you have more than one map to which you wish to apply the colour scheme. Alternatively, for a colour scheme unique to a particular shapefile, you could create a layer file (.lyr) but ensure that this layer file always stays with the shapefile, particularly when passed to a third party! The component RGB values (Red, Green, Blue) of each map colour could even be noted in a text file if other options are not available, ensuring that other users can re-create the colours in another software environment (a different GIS or perhaps a graphics package for publication). It seems a waste to design a colour scheme for a publication and then loose the ability to reproduce it at a later date.
 
Remember not to:
 
  • Create a colour scheme that is too complex to be interpreted. Although it is possible to use an almost infinite range of colours and patterns in GIS software, this does not necessarily make a clear map! Remember that it is not compulsory to apply a different colour or pattern to every feature having a different attribute. Grouping seabed habitats into similar types is a practical alternative. An often-used ‘rule of thumb’ restricts the number of different colour categories to approximately 15 because it is thought that is the maximum that can be easily distinguished by a user.
  • Symbolise a map using attribute codes that lack a common meaning (e.g., integer values to represent habitat descriptions), and then write text to describe the habitats as part of a legend. Instead, add descriptive attributes to the vector data resource that can be interpreted by other users (e.g. habitat descriptions or codes from an accepted scheme). Then the colour scheme can be applied to any attribute, while the important information is stored as part of the data resource.
  • Write lengthy text as part of a legend; it is extremely difficult to interpret the map if the legend is too complex, and the visual impact of the map may be compromised.
  • Use boundaries for features which are too dark and consequently obscure the internal colour of the features; this is often a problem for small features.
  • Design a colour scheme for a printed map without testing how the colours appear when printed; two colours appearing distinct on screen may look similar when printed (and vice versa).
 

Layout

Traditional elements of map layout (titles, text, legend, scale bar, logos) are sometimes omitted when creating electronic maps. However, they are crucial for a good map that is easy for users to interpret. Important things to consider are:
 
  • A north arrow indicating the orientation of the map;
  • A scale bar providing a visual indication of the size of features and distance between features on the map. The bar is usually divided into several parts and labelled with a map unit (e.g. km, metres);
  • Scale text to represent the scale of your map rather than a scale bar. Scale text indicates the scale of the map and of features on the map (e.g. 1/10.000, saying that 1 cm on the map equals 10 kilometres on the ground).
  • Resolution of the acoustic images
  • Coordinate system
 
The table of contents often takes the place of a map legend in GIS. If it is present in a map, it should have clear, intuitive names for data frames, layer names, headings, and class names. The table of contents is also a good place to provide map readers with additional information.
 
Links to other chapters
 
Links to other sections:
 
Links to websites:
 
 

All material variously copyrighted by MESH project partners 2004-2010

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