Smallworld Technical Paper No.2 - The Difference Between CAD and GIS

by Richard G. Newell & Tom L. Sancha

Abstract

Although there are some similarities between CAD and GIS there are many more differences. The most fundamental difference is that GIS models the world as it exists, whereas CAD models artif-acts yet to be produced. As a result the data that GISs manipulate is an order of magnitude larger and more complex than CAD systems have to deal with, and the nature of the data, its sources and its uses, are quite different. This paper compares the two fields in terms of their technology, data, market, user applications and vendor organisations.

Introduction

CAD is used to design new objects, which have not existed in the world before, whereas GIS is used to build a model of the world as it exists, including its history, in order to understand, analyse and manage resources and facilities. The data necessary to represent the world as it is enormously larger and more complex than the data necessary to represent new products This fact leads to there being major differences between CAD and GIS. Anybody who has worked in both fields will immediately be struck by the difference in emphasis between the two, basically stemming from the vast differences in data volumes and the fundamental raison d'etre of the two kinds of system. The following are typical lists of terminology and jargon that one might hear if one attends a conference on CAD or GIS:

CAD	GIS
drafting	graphical editor
layer	theme
solid modelling	terrain modelling
FE analysis	spatial analysis
NC manufacture	map production
drawing database	seamless mapbase
document management	version management
design system	information system

Even a superficial examination of the parts of GIS and CAD that one might think are similar immediately reveals some important differences, namely the graphics:

• CAD geometry is primarily constructed by a draftsman whereas GIS geometry is scanned, digitized or surveyed.
• CAD geometry contains many horizontal and vertical lines. Lines at regular angles are common. GIS geometry contains virtually no horizontal or vertical lines and, apart from right angles, other regular angles are rare.
• In CAD, circular arcs and curves are essential, in GIS they are virtually non existent. Some GISs do not even have a way of representing a curve, despite their frequent occurrence in urban areas.
• In CAD a typical polygon has few vertices, often four; in GIS a polygon may have many thousands of vertices.
• In GIS, operations such as mirror, rotate, scale and copy are unusual; on the other hand lines of a 'fractal' nature, such as contours and coastlines, are common.
• In CAD, schematic drawings, such as those used to represent electrical circuitry, are extremely stylised; but in GIS, the layout bears a close resemblance to the real world. However, the topology in both cases may well be very similar. There are a few forms of stylised map, a good example being the London Underground map, but these are not common.

The use of databases in CAD is often peripheral to the main task, and is commonly not provided by the CAD vendors. Databases are used to hold such things as catalogues of standard components and drawing registers, seldom are they used in the mainstream of the design itself save a few systems which handle the design of complex assemblies [1]. In GIS, the database is the most important aspect of the system [2].

The authors of this paper have designed several major CAD systems (e.g. PDMS Plant Design Management System developed by CADCentre Ltd., and Medusa developed by Cambridge Interactive Systems Ltd., now owned by Prime Computer Inc.). One of us is now developing an advanced GIS and has discovered that many fundamentals have to be rethought.

This paper describes many of the differences in, not only the technology, but also the applications, the market, the typical users and the vendors.

System Evolution - Darwin versus the Creationists

One of the earlier applications of computer graphics was to apply it to problems of design, typically in mechanical and electrical engineering [3]. In order to try and broaden their market presence, early CAD vendors applied their systems to mapping to produce what is now known as a "CAD mapping system" [4]. In order to be effective, major extensions were needed in the handling of geometry, symbology, and data volumes.

In general, CAD systems are geared to handle individual parts, they seldom use a database except as a loosely interfaced adjunct on the side to hold catalogues or manage drawings. So in order to try and address the data volume problem, the next extension of CAD mapping systems was to hook them up to a proprietary database to try and produce what is known as a GIS. Unfortunately, this is trying to stretch the original CAD technology too far.

Early computer graphics was seen as an exciting new technological development with wide potential in many fields. Some had to wait a long time before realising the benefits of that early vision, and it was in the process of this evolution that the subject diversified into a wide range of different applications. From early point and vector graphics came the first electrical drafting systems, which then evolved into 2D mechanical design systems. The 2D systems evolved into 3D wireframe systems, which because of their roots soon became obsolete. What was really needed was not the Darwinian approach, but the Creationist approach, since as we all now know, solid modellers appeared from an entirely different beginning.

The process of evolution is ideal for refining and improving an initial concept, but if the concept is wrong or has important short-comings, then there is a limit to what can be done. Unless a radical new beginning is made, an evolving system can become over-developed, software fatigue will set in and eventually the product will die, because better alternatives will emerge from elsewhere. It is a shame that users are not more courageous about moving from one system to another, because if they wait too long, they are only stacking up far worse problems for themselves in the future.

Owning a CAD system is rather like owning a dog, the prudent owner knows that the dog is likely to die at about the age of 12 or so, and therefore when his dog is about 8 or 9, he acquires a new dog, so that when the time comes, his new dog is well trained, has become part of the family and the grief of losing his old dog is minimised. (This parable is not original, it was related to one of us by Roger Breuleux). If one wants to see a more relevant example, just witness how the world is switching its software development from Fortran to C.

What has all this got to do with CAD and GIS? The early CAD vendors assumed two things: first that utility companies, municipalities, environmental resource agencies etc. required mapping systems; and second, that these so called mapping systems could be evolved from their electrical and mechanical CAD systems. Well, they were wrong on both counts: these users require GIS systems which cannot just evolve from CAD systems.

It is common place these days to debase new terminology. How many database systems that handle tabular data label themselves "relational"? How many systems that have some concept of an object call themselves "object oriented" and how many graphics and CAD systems that can put a map on the screen of a CRT call themselves "GIS"? Building a GIS requires a radically new approach technologically. The state of the art systems now come from specialist companies, not from the major CAD vendors.

Technology

The normal approach to tackling a major design project with a CAD system is to segment the problem into manageable pieces. Each piece corresponds to a part, a subassembly or a main assembly, the limitation being the amount of data that one designer can hold in his head at one time and then get on one drawing. Partitioned databases seem to be the order of the day. Such databases may be managed by an additional continuous database which performs the function of document management. In most design situations, the problem is straightforwardly segmentable and therefore document management techniques are appropriate for handling such functions as drawing issue, approval, archive, revision and change control. One area where this approach breaks down is in plant design which, due to the 3 dimensional complexity and the difficulty of segmenting the problem either by discipline or space, requires a continuous database. However, most modern plant design systems segment the database, which makes the system much easier to implement for the CAD vendor, but much less convenient to use for a large design team.

GIS systems which have evolved from CAD systems work with partitioned databases, corresponding to map sheets, but this is not what the user requires. Our world is continuous, there are extremely few places where you could naturally segment a continuous landscape into pieces and therefore the GIS system must always work on a continuous database. Further, some attempts to extend a CAD system into GIS involve hooking the CAD system into some third party database management system. Unfortunately, today's generation of database systems does not address all aspects needed for a GIS. One might hope that marrying together two inadequate technologies would solve the problem, however, these two technologies do not integrate too easily. In such systems, the data management facilities of the database system are not fully accessible from the CAD system command language nor are the graphics facilities of the CAD system available from the database query language.

Whereas CAD is based on a database of parts, managed by a document control system, GIS should be based on a continuous database management system with integrated graphics facilities. If one had to choose, there is a better chance of extending a database system into a GIS than extending a CAD system into a GIS. Partitioned databases do make life much easier when trying to develop new systems. Continuous databases introduce some particularly difficult problems:

• Performance
• Concurrent users
• Efficient spatial retrieval
• Multiple version management

In GIS these issues must be addressed, save for a small number of niche applications of limited scope.

Data Capture

In CAD no sensible company would insist that the whole of their existing drawing archive was input to the computer before they could start work. With a CAD system you can start working with a blank sheet of paper. In GIS, you cannot even start before you have an adequate mapbase. People will argue as to what is regarded as "adequate", (e.g. raster backdrop versus vectorised versus full topological model), but all will agree that some form of mapbase completely covering the area of the GIS is essential.

Some people think that it would be the mechanical designer's dream if a machine existed which was capable of turning his drawing archive into a CAD database automatically. Assuming that the resulting database is to have value beyond merely the replacement of the paper archive (which may well be meritorious in its own right), then there is a requirement that the database must contain elements at the highest possible level, ideally objects. This means not only vectors, circles, arcs and curves, text strings, and symbols (all of which are now looking practical to achieve), but also topology and geometric precision. These latter two are essential if the CAD drawing is to be used for anything other than editing it and redrawing it again on paper. This might include parts listing, connectivity analysis and generating manufacturing information.

There is a parallel in the problem of trying to capture a map base which is to be incorporated into a GIS, but again if only vectors, text and symbols are recognized then the resulting map database has little use beyond that of a backdrop. Again, the meanings of lines need to be captured together with their topological relationships in order to be able to represent point, line and area features. Further, there is the additional requirement of being able to associate these geometric features with other real world data held in the database. It is interesting to compare and contrast these two apparently similar problems. It could be true that up to the point of obtaining vectors, text and symbols the two problems are similar, (which is probably why many scanner and vectoriser vendors address both markets) but the methods of obtaining topology and precision are different.

A typical map has many more differing line styles than a mechanical drawing. Each line in a map will correspond to one or more features whose codes need to be associated with it. The identification of topology in a map means recognizing point, line and area features and how they relate. In the case of mechanical drawings, lines need to be collected into objects, such things as dimensions need to be assembled and then associated with dimension points.

In GIS it is often the case that the precision of the lines on paper, when properly corrected for paper distortion by means of software rectification procedures, is good enough for inclusion in the database. This is rarely the case with mechanical drawings. The precise geometry is specified by the dimensioning and other annotation and not by the lines of the drawing. This information has to be extracted by sophisticated techniques such as variational geometry it is usually more efficient to recreate the drawing via a suitably tuned drafting system.

Both CAD and GIS systems have a requirement for geometric construction, and in the introduction to this paper we described a number of the differences in emphasis. However, in GIS there is an additional difference in the processing of surveyed data. Here it is not just a simple matter of applying Euclidean geometry, because surveyed data usually contains deliberate redundancies for cross-checking purposes. Closing a traverse and triangulating a new survey point from more than two existing points are common examples. For small scale work, there is the additional complication of taking account of the earth's curvature.

Although the widespread application of CAD technology is about ten years ahead of the application of GIS, bulk data capture for GIS is far more widespread than it is in CAD. However, this may change as the realisation dawns that CAD is but a small part of the overall requirement to handle design documentation and technical publication in the large. The use of scanned, unvectorised drawings is now considered to be an acceptable approach in systems such as desk top publishing. It is quite likely that systems of the future will address the overall problem of technical and design documentation using inputs from whatever technique seems appropriate, including scanned input, desk top publishing, word processing and of course CAD.

A unique characteristic of map data is that it is in many cases inaccurate and fuzzy. Often a map base will contain data of different standards and qualities. Sometimes the absolute positions of things are not known accurately, but relative positions may be known precisely. For example it may be known exactly where a pipe is buried in a road, even though the road centre-line is only known approximately.

All this leads to an important requirement for the GIS of the future to hold quality metadata in the database. Such metadata will describe the source, accuracy, reliability and completeness of all data in the database. Without this, a GIS in the wrong hands could become a very dangerous tool. CAD does not seem to have anything remotely resembling this requirement.

In CAD databases we are faced with the inverse problem, the database model of an object may be exact, but the real world manifestation of the object can only be an approximation to the database, the limits on the amount of variation allowed being specified by means of tolerances.

One whole area unique to GIS, but having no equivalent in CAD is remote sensing, for example by means of satellite. Here, the problem of capturing the data has been solved, the question arises as to how to handle the vast amounts of it that rain down upon us from the heavens. For example to cover the UK at a resolution of 30 metres (e.g. Landsat) requires about 300 million data samples and that's just one coverage at one time. The image processing techniques for handling this data have not found much application in CAD yet.

Lastly, we mentioned at the beginning that GIS systems are not only meant to model the existing world, but also its history. This comes under the heading of handling temporal data, which is still a research topic [6]. CAD has an allied problem in version management, which it usually neatly sidesteps by pushing the problem into the document management system. This of course is possible if the database is partitioned into documents, but is more difficult if one has a large, continuous, multiply accessed design database.

Applications

The emphasis of CAD is on the creation, modification and documentation of design information. The emphasis of GIS is on modelling, analysis and management of geographically related resources. CAD is used to create a model of something that does not yet exist in order that it can be made; GIS is used to create a model of the existing real world in order that it can better be understood, that we can monitor change and plan for a better use of our earth�s resources in the future.

Whereas CAD is used to design and create many of the products that we humans use to destroy this planet, hopefully GIS will be a tool for environmentalists to use to prevent this disaster. This is a CAD journal, so most readers are familiar with applications of CAD, if not then they only need to read a few back numbers. As the readers may be less familiar with GIS applications, we list here a few which illustrate the potential of the technique:

Birds - The Royal Society for the Protection of Birds were interested in finding out the distribution of a small wading bird, the Dunlin Calidris alpina, on Scottish moorlands. They had commenced a ground survey of moorland birds and came to the conclusion that, given the resources available, it would take 25 years to complete. A preliminary investigation which showed a correlation between known breeding grounds and just one spectral band of Landsat data led them to investigate other potential areas. Despite initial scepticism, RSPB representatives investigated some of the early predictions, with the pleasant surprise that a number of them were well populated with breeding Dunlin. The potential of this technique for predictive modelling is obvious.

Vineyards - An organisation in Spain wished to plan the best locations for future vineyards. Vineyards can only be planted on land that is available for agricultural use, where the soil type is suitable, the ground slope is within certain limits and the aspect is towards the sun. This problem is solved by taking separate map overlays (themes) representing land use, soil type, ground slope and aspect to perform what is known as an overlay analysis so that all areas with the desired properties can be identified.

Water - A water company receives a number of reports from customers that they have low water pressure. The water company has a GIS containing a complete representation of the network together with the relationship between all branches of the network and the consumers' addresses so the system can make an accurate prediction of the location of the pipe burst.

Planning - A planning department receives an application from a house owner to extend his house. One of the initial tasks is to find the address on the map, from the map all other property owners that might be affected are deduced, and a letter to each of them is drafted. The planning application can be compared against other applications, master planning schemes and urban classification zones, as well as checks for possibly affected services such as water, sewerage and electricity.

The above applications of GIS are typical examples which serve to demonstrate the entirely different nature of GIS from CAD. In short, GIS is much more of an information retrieval and analysis system than it is a system to create new data.

The Market

If one asks a CAD marketeer to classify the market according to different industry sectors, then he will come up with categories like:

• Aerospace
• Automotive
• AEC (a confusing and vague term)
• Electronics

If one asks the same question of a GIS marketeer, then you will get the following:

• Utilities
• Local and regional government
• Environmental resource agencies
• Transportation
• Cartographic institutes

Within individual organisations of course there may be a requirement for both technologies, but this will frequently be in different departments. For example, the planning department may use a GIS to plan the location of a new hospital, but the architects department would use a CAD system to design the hospital.

The Vendors

It is true to say that CAD system sales have polarized. On the one hand there are standardised low price PC products sold via large distributor networks and on the other there are high priced, customised special purpose CAD systems running on more expensive hardware. The latter kind of sale requires a much greater degree of consultancy, customisation and long term relationship with the customer than the former. The days of the expensive standard system are nearly over, save for those users who must maintain compatibility with their existing investments.

The distribution structure of PC based products is different from sales organisations of larger systems. The software is usually distributed by a network of dealers who make the actual sale to the end user. The dealer is dependent on the hardware component of the sale in order to maintain his margin. He will be dis-inclined to get involved in lengthy education, training and customisation since the amount he can afford to spend on each sale is limited. His geographic area is usually relatively small, and he will not be focused on the particular product. He will usually be selling other CAD products and also a wide range of hardware as well. Some distributors may have a separate division in order to provide customisation services, but this is often not the case.

Large system sales require a different approach. The customer requires extensive benchmarking, often involving some preliminary customisation. The sale is accomplished by direct selling, often to a fairly high level in the company, the sales team must contain highly qualified people who are capable of understanding the broader requirements of the customer in order that they can translate into a customised system at the end of the day. In order to make customisation economic, the requirements of the product itself in providing tools are more demanding.

The vendor organisation of a GIS system is nearer to the one that is now required for selling up market CAD systems. That is not to say that there are no PC GIS systems, there is clearly a market for these systems which can tackle certain niche applications of limited scope. However, it will be a few years before PCs can handle the data volumes and CPU requirements of a full blown GIS. Today, implementing a GIS in a new organisation can be a very large task, which will involve the vendor for far longer than the typical CAD installation. The GIS vendor will need to provide extensive consultancy and customisation in the areas of data modelling, capturing the map base and interfacing to existing (sometimes very large) database systems, which implies extensive systems integration. Few CAD vendors are geared up for this scale of effort.

Conclusion

The CAD market is now mature, but the GIS market is still in its infancy. Thinking that GIS is simply digital mapping, several of the established CAD vendors tried to adapt their CAD systems for GIS applications. This resulted in most unsatisfactory compromises.

Despite the great expectations of CAD revolutionising the design processes, today most CAD systems are only used to produce drawings and pictures. Indeed the market is dominated by one PC based drawing system. This paper has shown the differences between the technology, the data, the applications and the usage of CAD and GIS systems. Will the evolution of GISs follow the same lines as the evolution of CAD and, if so, what will be the surviving core that is most used?

It will be interesting in the future to see how each technology will borrow from the other. One application, facilities management, could be considered to be positioned somewhere between the two, and future solutions will need to borrow from both, particularly the ability to handle large databases and seamless mapbases from GIS and the design capabilities (sometimes 3D) of CAD.

The CAD users of today now have their drafting systems and their modellers and the main problem that they now face is the management of the large amount of design information generated as well as the integration with manufacturing and other corporate activities. This is a problem in managing and coordinating large distributed database systems, a problem also encountered in GIS [7]. References to the shortcomings of today's commercially available database offerings are all too common in the literature now [8], this means that new generations of database will appear in the future to meet the challenge.

The requirement of a GIS to be modified and customised will force the pace of development of much more powerful software tools. The object oriented approach to structuring major software systems is becoming much more widely understood now, even though commercial systems embodying the approach are few and far between. We are beginning to see the arrival of systems which attempt to address the database issue in an object oriented environment. This technology will enable a quantum leap to be made in both GIS and the CAD systems of the future.

References

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