Lecture 3 - Computer cartography

Lecture 3 - Computer cartography
This lecture will cover:

Essential map furniture:
 North arrow
 Scale bars
 Legends
Choosing colours:
 Colour spaces
 Help with choosing a colour scheme
 Colour blindness issues
Symbology and layering
Issues arising in cartography:
 Legibility / print quality and keeping it simple
 Copyright
 Subconscious messages
 Honesty about data quality
Metadata and archiving
The future of GIS in archaeology:
 The third and fourth dimensions
 Other areas of research

Essential map furniture
Today we shall be talking about the production of maps that both look handsome and make sense to your readers. Cartography
has been greatly eased by the invention of GIS, but still remains an art form that takes time and thought to master. We shall begin
with the three essential elements of so-called map furniture. Firstly, every map should include a north arrow (or grid) and a scale
bar. Scale bars should be in appropriate units to the scale of the map, and set to use sensible round numbers. Further, it is better
to use a scale bar than a numerical rendering of map scale (such as 1:25,000), as you cannot be sure that a map will not be resized
when inserted into a document or when copied after it leaves your direct control. With a scale bar and north arrow, it should be
easier for readers of your map to then orient themselves and get a sense of the size of your region. It can also be useful to include
a small inset map to show whereabouts your study region lies, but this is added most easily in an external image editor.

Secondly, if you want readers to be able to interpret your map, you need some form of explanation of what it features. If used in a
document, you will obviously describe the map contents in the figure caption, but it is also usually best to provide some sort of
legend to illustrate the same visually. The legend construction system in ArcGIS is rather complicated and it can take quite some
time to produce results that are legible and also coherent across a set of maps. Furthermore, if you have a lot of layers in your
map, it is all too easy to fill the legend with unnecessary clutter. As such, best practice is probably to include just the layers that

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relate to your interpretation of the map in the legend, and any other layers where it is not obvious what the symbols actually relate
to. You should certainly include any layers in the legend that vary by quantity or by some category.

Colours
The choice of colour schemes is a surprisingly complex business. It is very easy to produce horrifically garish looking maps,
especially if you do not alter the standard ArcGIS colour schemes. Essentially, colour choice should follow one of two paths:
attributes that vary numerically should be displayed along a colour scale; attributes that vary categorically should be displayed using
contrasting colours. Colour can be conceived of as a cone that varies in hue around the edge, lightness vertically, and saturation
travelling in and out of the centre. Varying lightness when attempting to use multiple hues will quickly cause your map’s colour
scheme to plummet into the depths of mathematically different but visually similar murky greys. Varying saturation is excellent for
conveying messages about intensity, with the paler colours intuitively conveying lower numerical quantities. Varying hue can show
quantitative change (through gradual movement around the cone) or categorical change (through picking colours that are widely
spaced across the cone). It is possible to convey two attributes at once using colour, by varying category according to hue and
quantity according to saturation.

Websites such as Colorbrewer can help you to choose a good colour scheme that conveys the messages you wish it to convey.
Wise use of colour can help a user to intuitively understand what it is that you wish them to take in. As a simple example of this,
colouring a DEM by elevation so that it varies from greenish tones, through brown and up to white should prove familiar to most
readers, as that is the way that they will have seen atlases coloured since childhood. However, as an example of how this useful
cliché could also be misleading, white tones echo with the metaphorical yodels of the Alpine massif and would look rather silly when
used to illustrate the altogether less heady peaks of East Anglia, as would be the standard setting when using one of the preset
ArcGIS elevation colour schemes.

Another major stumbling block with colour scheme choice is caused by colour blindness. Up to 10% of the male population (as it is
caused by a defect on the X-chromosome) are red-green colour blind, meaning that poor selection of colour could easily make your
maps nigh illegible to around 5% of your potential readers, including at least two eminent Roman archaeologists that the author
knows of. Other forms of colour blindness are far less common, but can still cause problems for certain readers. Essentially, you
should avoid using any colour schemes that oppose red colours against the green-blue portion of the spectrum. Unfortunately,
many of the standard colour schemes built into ArcGIS fall squarely into this trap. It is best not to ignore this issue: you could
always try talking through your maps with a colour blind colleague to see how visually comprehensible he finds them.

Symbology and layering
Choice of symbols is a less complicated area, but poor choice can still lead to very ugly maps of unprofessional appearance.
Symbol size can be used to convey numerical variation, and symbol shape used to convey categorical variation. As should prove
apparent, by varying colour, size and shape of symbols, you can convey an awful lot of information in a single map. Different
symbols can speak different messages, not all of them good. For example, using crucifix symbols to depict religious sites of all

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types will very quickly get you into trouble with many readers. Alternatively, prior to the 1930s, a swastika would have made a fine
symbol for a Buddhist temple, but using swastikas in your maps would tell a rather different story today. In that situation, the use of
neutral shapes would be best practice. ArcGIS possesses a vast multitude of different symbols, to depict a vast multitude of
different types of geographic object. However, if you wish to produce professional-looking maps, it is generally best to stick to a
small number of simple shapes (circles, squares, triangles, etc.) and only increase complexity when necessary.

When dealing with polygon data, such as survey transects or field systems, the results may be presented as either a choropleth
map, by using dot densities, or by the use of a central symbol. Choropleth maps are maps in which polygons are coloured
according to some attribute. Subject to the consideration of colour choice discussed previously, they are usually the simplest way of
conveying information about this sort of data. Central symbols provide another useful option, especially for black and white printing,
but can be a little confusing if it is not made clear that the symbols represent the whole polygon rather than just its centre. Dot
density maps are underused these days, but can provide an excellent and very elegant way of displaying several variables on a
single map, if kept simple and clear.

Careful ordering of the layers in your map is also important prior to the production of any final output. For example, a river layer
placed above a road layer will look rather silly (unless aqueducts are prevalent in your region). Generally speaking, it is best to start
with contextual layers at the back and move upwards in a logical order until you place your most important layers on top. Also, just
because you have a large number of map layers, does not mean you should switch them all on in your final maps. A DEM might
provide excellent context, but if it elides the message being conveyed by your upper analytical layers, it may be better just to save it
for an initial context map, then switch it off for your substantive result maps. Therefore, best practice is just to use the layers that
are important to your message and interpretation, with any context being kept to the necessary minimum. Careful use of partial
transparency can also enhance the story being told by your maps.

Cartographic issues
We have already touched on some of the particular issues that can arise when dealing with cartography, but there are several
others that need to be discussed too. Returning to colour, further problems arise when thinking about printing and publication.
Colours on a black computer screen are built up (additively) from black using red, green and blue prime colours. Black is no colour,
white is maximum on all three channels. However, colours printed onto white paper are subtracted from the white using magenta,
cyan and yellow prime colours. In this case, white is no colour and black is maximum colour of all three types (good printers use
black ink too for colour printing, as maximum yellow, cyan and magenta actually looks quite brown). As a result of this, printed
colours can look quite different from the same colours on the screen, especially if using a lower quality printer. It is, thus, best to
experiment before printing off too many maps. Secondly, cost considerations also come into colour printing. Whilst colour is free
on the screen (and thus for maps posted to the internet, etc.), it has a considerable cost when it comes to printing. This is
especially the case when it comes to submitting articles for publication, as many journals may only run a few colour pages (although
they may allow more colour images for web versions). Therefore, make sure you consult with a journal’s guidelines for authors
before producing your final maps. In general, it is best to keep things simple: this is a good maxim for all cartographic work.

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Copyright is another thorny issue when it comes to publication of maps. In the UK, the licensing of Ordnance Survey data is
complex and rather confusing: guidelines to the use of OS data can be found on the Digimap website. Google Earth imagery is
licensed for not-for-profit non-textbook academic uses, so long as proper attribution is given of their rights. You must make sure to
quote the copyright of any copyright holders for any layers in your map on your map. This can include yourself if you so wish.
When dealing with not-for-profit publication, that will usually prove sufficient, but it is always best to consult with copyright holders to
see what they wish you to do.

We have already encountered the issue of subconscious messages conveyed by your maps. Choice of colour can convey hidden
messages, such as the presentation of empires on maps of the world in blood-curdling red tones. Choice of symbol can also cause
unforeseen offence to certain parties, as previously discussed. Clichés can be a useful way to express a message that is familiar
from countless previous maps, but they must be used wisely so as not to tell false tales. All maps deceive in some way, as they are
inevitably a simplification of the real world, but we must be careful that the deception we present is an honest one. As such, you
should always be explicit about any concerns you have over data quality in published maps. Choice of specific symbols (such as
the use of empty and half empty symbols) or clever use of transparency can be used to provide a visual clue as to any uncertainties
in your data, but should always be accompanied by an explanation of any concerns in the figure caption. It is easy to use a cleverly
constructed map to hide inadequacies in your data, but to do so is dishonest.

Metadata and archiving
Finally, if you wish your data to be accessible to other researchers, you must think about archiving and metadata. Archives such as
the Archaeology Data Service (ADS) collect large amounts of computerised archaeological data and maintain an updating system to
make sure that it remains accessible to future generations. However, to submit data to such an archive, you must make sure you
meet their metadata requirements. It is important to record metadata for your data in any event, as without it assumptions that
might seem clear to yourself may be overlooked by any future researchers.

The main things to record as metadata would include:
A title.
The date at which the data was digitised.
The source.
The software used and data format.
Who did the work.
Any concerns about data quality.
Any recorded precision or accuracy measurements.
The geographical projection and spatial extent of the data.
A descriptive abstract of the data and its purpose.
Any access or usage constraints.

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The data’s status, i.e. is it complete or a work in progress.
Details of any updating schedules.
Referencing information.
Dataset language.
Keywords and any relevant topics.
All this and more is included in ESRI’s metadata format for shapefiles. If you wish your data to be usable by future researchers, as
good academic practice dictates, then you should make sure to record as much of this information as possible. Detailed metadata
is the key to the survival of digital data: it may be a chore, but it should be a mandatory task.

The future of GIS in archaeology
Naturally, GIS is a dynamic subject and always developing in myriad ways. Many of these new avenues of research will come to be
applied in time to archaeological material, and some are being generated from within our discipline. The biggest future change in
GIS is the move into full three dimensionality. This is a process that has been going on for quite some time, and is approaching
becoming a fait accompli. GRASS is already fully functional in this regard, and ArcGIS is getting closer to the same goal. This is an
important improvement in the way GIS is able to model the real nature of the world. Further away into the future, however, is the
move to fully four dimensional GIS, i.e. GIS that also take into account time as well as space. Temporal GIS or TGIS remains
primarily a research topic, with what little commercial software there is available very restricted in terms of function. Animation of
maps, including for web delivery, is one area in which TGIS is beginning to make itself known and functional, but true spatio-
temporal analysis is still in the early stages of development. Finally, there are many other areas in which GIS continues to develop,
including improved methods for viewshed and least cost path calculation, and also the use of GIS to study non-spatial variables in a
spatial environment. GIS is an exciting field to be a part of, with new tools being developed all of the time, and it is well worth
absorbing oneself in the community to see what fresh insights these forthcoming solutions might bring to your ancient data.

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