Proceedings of the International Hydrographic Conference’Evolutions in Hydrography’,Hydro06
Antwerp, Belgium
6-9 November, 2006
BEYOND ENC
S Salter and A Watkin
Simon Salter, CherSoft Ltd, The Point, Bradmarsh, Rotherham, UK S60 1BP
simon@chersoft.co.uk
Andrew Watkin, CherSoft Ltd, The Point, Bradmarsh, Rotherham, UK S60 1BP
andrew@chersoft.co.uk
This paper discusses contemporary issues with ENC data and display systems. It
identifies several areas where experience has exposed deficiencies in the use of
ENC. We then go on to suggest several ways of improving cartographic data
handling in the marine environment.
Some of what we argue may seem counter-cultural since our experience of software
development in the ENC context has given us an awareness of its limitations as well
as its advantages. We want to see ENC continue to develop and for this reason we
feel that it may be time to take a fresh look at some issues before they become
needlessly ingrained in our collective consciousness.
There are a number of issues that fall into this category. In this paper we discuss just
two: the importance of cartographic input to a charting system, and the potential
advantages of a more dynamic and situation aware approach to symbology and
standards.
1. DRAWING ISSUES
1.1.
Vector and Raster
Vector charts (ENCs) are generally considered to be an advance over raster
charts (such as ARCS) and possibly even a successor to paper charts. However as
they come into more common use several generic deficiencies are becoming
apparent. Much has been said about the relative merits of raster and vector. Possibly
too much since a large proportion of the voices have been driven by political and
commercial concerns arguably to the detriment of the end user. However there
remains an interesting technological issue which is seldom dwelled on; raster charts
look better.
Fig 1: Typical raster chart. Clear understandable information which has carefully
been compiled so as to be fit for purpose
In our experience vector evangelists rattle out a litany of raster failings at this
point, but the fact remains - raster looks better. It is clearer, suffers less from clutter,
is better balanced and is totally familiar1.
So what’s going on? It seems obvious to us that the fundamental difference
between raster and vector as implemented by ARCS and ENC is the contribution of
the cartographer. While a good raster chart is an excellent facsimile of the chart a
cartographer creates ENCs disregard much of the essential layout information since
they try to recreate the chart dynamically, redrawing it from scratch (or very nearly).
If we take a step back here it should be no surprise that this automated process is
less effective than a cartographer’s direct output. Good cartographers are highly
skilled people who exercise professional judgement and experience in what they do.
They can spend a day looking at an amended chart trying to assess whether it still
retains the same value in terms of balance, content and clarity – and even scale and
coverage. Everything we know about computer automation should have warned us
that the deep human complexity of this process would be difficult to reproduce.
Computers are dumb, don’t learn from experience and have very little time in which
to do the drawing. Substituting algorithmic complexity for cartographic expertise for
the most part fails miserably and our first thought for the future is that chart formats
will need to retain more input from the cartographers.
1
Neither is this article designed to be too one-sided in this particular aspect of the debate. CherSoft
has worked very effectively with both raster and vector charts for 12 years.
1.2.
Rendering
Rendering of ENC is comparatively slow and rarely gives satisfactory results when
compared to a paper chart. Creating good quality charts from vector data will require
adding cartographic information to the data set. There are some hints at this in ENC
and a few more in VPF. The notion can be taken a lot further. Basically chart data
should be designed for display at a particular scale.
All good map or chart data is designed for display at a particular scale, the scale
in turn chosen by the needs of the end-user. This is as true for hill walkers and car
drivers as it is for mariners. Yet with ENC, a combination of the limitations of display
technology and inside-out, feature driven development, has meant that the balance,
usability, and at times safety of an ENC has been compromised. We cannot put a
standard size chart on one inexpensive screen so we work around the problem by
panning and zooming. Good cartography has been sacrificed in the temple of zoom.
Fig 2. Vector data. Without the ability to zoom displays can easily become
cluttered
Moreover, zoom is not necessarily the great advantage of vector charts that some
would have us believe. It is closer to the truth to acknowledge that ENC actually
needs zoom to help manage clutter. At anything like the information density of paper
charts the ENC becomes dangerously cluttered.
Zoom is the obvious way out, but it does then dump unnecessary responsibility on
the mariner. Instead of being presented with the right data in the right form and at the
right scale, the mariner has to make additional decisions about what he wants to see
displayed. Some may welcome this, although the safety implications are obvious.
The theoretical basis of S-57 is in the separation of geometry and attribution. This
produces a (simple) model of the earth. Other mapping schemes use a database
driven representation of a map. This has substantial cartographic information in
addition to the model of the earth. Land map sales are more competitive than marine
charts and so have maybe been pushed that bit harder. The lack of cartographic
information in S-57 compounds the rendering problem so that the information density
has to be kept down.
At an imaginative stretch you might find a use for 1:53,146 scale, or some other
arbitrary scale, chart but for the most part knowing and understanding the scale of a
chart is essential. Given a series of fixed scale charts the cartographers (remember
them?) can be rolled out to turn the raw survey data into useful chart information.
Their task involves aspects such as deciding what should be visible, how labels
should be placed, what symbols should be used and so on. The chart has a purpose.
Manipulating the data towards this end is highly skilled job. At the culmination of this
we have another cartographic data set which tells us how to layout the chart at a
certain scale. Great. Now the geeks can get involved. They will pre-process the data
for display, optimise the rendering engines, cache off-screen images and support
dynamic anti-aliasing. What ever all that means the end result will be fast, clear,
reproducible chart images with all the benefits of an active data layer and the clarity
or real paper charts.
2. MAKING FULL USE OF THE AVAILABLE DATA
2.1.
Data Quality
ENC predicates better quality data than is actually available. Work is underway to
re-survey many areas of the world but it takes time. Currently much ENC data is
produced by tracing over existing raster charts. Because ENC cannot capture all the
information and knowledge that goes into a paper chart it cannot make full use of the
information actually available today. Paper charts used to be produced, in many
respects, in isolation from other paper charts. The essential requirement being that
the data was consistent on the chart – not that it should be consistent with adjoining
charts. This data is still being used for ENC which is why you will often see
discontinuities such as contours jumping at cell boundaries. On paper charts the
Source Data Diagram (SDD) is an essential consideration for the experienced
mariner. ENC has the CATZOC attribute but this is of limited use. Rather than trying
to hide issues of data quality it should be made an important aspect of the chart
enabling the cartographer to make full use of the data to create a chart without
misleading the navigator.
2.2.
Projections
Although ENC claims to be projection independent this is not quite true. The path
of a line between two points is not defined in ENC, although it is always taken to be
straight line on a cylindrical projection. Co-ordinates are therefore defined on a two
dimension grid (the surface of the cylinder) and so the ENC model is a flat, square
world. Although navigators are typically comfortable with the distortion associated
with cylindrical projections, these effects are really a hang over from paper based
charts2 and this is an odd juxtaposition with the notion that it is a good model for
marine data. It is possible, but computationally intensive, to re-project ENC data, but
other approaches to representing vector information might be more conducive to
supporting general purpose systems such as would be essential for high latitude
navigation. This is important since with the advent of global warming trans-polar
routes are becoming increasingly feasible and we cannot ignore high latitudes to
anything like the extent we once could.
Once we free ourselves from the limitations of paper charts the most obvious way
to view the world is to look at it like a space man. Or a bird. This is what the world
looks like if I were flying over it. Technically we call this a tangential projection. It is a
natural way to look at the surface of the planet. By actually defining a straight line
and using 3D coordinates a successor to ENC could power a new generation of
chart viewers.
Fig 3. Google earth – a tangential projection which displays high latitudes as
easily and naturally as equatorial regions
Another type of useful projection is a variation on the bird’s eye view where the
direction of view is angled up towards the horizon. Again this is a very natural view in
that it mimics common human perception. Objects in the distance, where they are
less important, become compressed where as closer objects appear larger. Just like
real life. This type of view is already commonly used in automobile satellite
navigation systems – which of course have to be readily useable and
understandable to untrained navigators.
2
ENC is still tied up with paper charts and so inherits a whole raft of issues including apparently
strange behaviour at high latitudes.
3. DYNAMIC SYMBOLOGY
The potential advantages are obvious if we think it through. While the main drive
behind symbology in ENC is one of standardisation (one object, one symbol,
everyone knows what it means), this makes much less sense when one considers
the role of a chart in terms of situational awareness. For example, on a bridge
system a nearby ARPA target heading straight towards you at 40 knots should slap
you in the face. It is important, you need to be very aware of it, and quickly.
Meanwhile another target at the limit of radar range doing 4 knots away from you is
less of an immediate concern. A standard symbol cannot make this differentiation.
Alarms on the bridge are almost useless because of the competition from all the
other alarms, so an event like crossing a Safety Contour (a second very real
example) needs to be signalled to the navigator in a more connected way. By this we
mean that the symbology should help to show immediately what the problem is
rather than just indicating that there is a problem somewhere.
The appropriate approach to situation awareness depends on purpose. In a
passage planning exercise an interactive list of potential hazards is quite
appropriate. However in a front-of-bridge voyage monitoring situation a more
automated display that concentrated on immediate issues would be more useful.
Facilitating a rapid appraisal of complex scenarios requires a much more dynamic
approach to symbology. This does not just mean being able to switch layers on and
off; it means variable size, colouring, transparency and even animation within the
symbology. It means being able to manually mark objects for specific purposes,
allowing dynamic input data (GPS, ARPA, RADAR), modifying the display and
allowing the navigator to be able to interact by adapting symbols to suit his purpose.
(The ENC safety contour is a step in this direction albeit a very small one).
To create real dynamic displays will require a re-think over the way standards are
managed. Criteria will need to be phrased in terms such as ‘is this situation clearly
visible?’ rather than the prescriptive testing for the appropriate static symbol. At the
same time information such as display scale and source data information need to
modify the drawing so as to create a feel for the quality of the chart information (back
to the cartographic point). Contour lines and other symbols should be drawn using
less precise edges where appropriate to offset the usual human tendency to assume
that computer data is completely accurate.
Fig 4. Simple dynamic symbology. The safety contour is an immediate concern to
the vessel and has been high-lighted in red
Full situational awareness requires access to an open ended set of data and
hence symbols. The full range of possibilities is large and growing: Radar, ARPA,
AIS, tides and so on. To try and regulate all the possibilities would require an
extraordinarily capable crystal ball. So if type approval is not intended to inhibit the
development of these technologies then the approval criteria will need to be coined
in terms of subjective usability.
There is a lack of stability in ENC caused by the number of parameters required to
specify a view. For paper charts the only required parameters are chart number and
update week number. This defines completely and reproducibly exactly what can be
seen. However for ENC we need a list of contributory cells, zoom level, rotation,
position, screen size, screen resolution and a hundred or more user configurable
display options. The combination of these produces a chart display which can be
disorientating and unpredictable. An incident investigation, for example, would want
to ascertain exactly what chart information was being displayed. Easy with paper
charts but very uncertain with ENCs. Much of this instability is an attempt to deal with
clutter. The ability to switch layers off is really a de-cluttering control which then
dumps responsibility as to what should be displayed onto the mariner. However the
chart display should really be a tool not a process. Cluttering in charts is a
combination of static symbology and lack of cartographic input. If these are dealt with
appropriately then there will be far less need for operator intervention in creating a
useable display. Charts will be more stable, more familiar, more useable and safer.
4. SUMMARY
Computers cannot build good charts from raw survey data. Future chart
information needs to include cartographic expertise.
Rapid situation awareness is a key issue in terms of the advantages of dynamic
symbology and more intuitive projections.
Neither of these issues are new, but we should be examining them more closely in
the context of ENCs. It is a mistake to think that the way ENCs are produced and
regulated now will be fixed into the future.
5. CHERSOFT
CherSoft is a small UK based company that has been specialising in navigation
systems with high performance raster and vector display kernels for the last 12
years. www.chersoft.co.uk