Digital Surveying across a river

All sugar estates need to be mapped accurately for land title purposes and for management control. Advances in technology have allowed cadastral and topographic mapping to be produced digitally with many potential advantages compared to traditional manual cartography.

This paper outlines the principles of digital terrain modelling and gives examples of its use, particularly for mapping, on a number of sugar estates. The output of a commercially available model, demonstrates the general concepts of digital mapping and possible future applications are discussed.

Digital terrain modelling is a method of representing the ground digitally and in three dimensions. Modelling is very useful in agriculture as the third dimension provides information for such considerations as drainage and calculating quantities of materials.

Digital mapping is one of a number of possible outputs from a digital terrain model. Although plots are in two dimensions (2D), data can be in three dimensions (3D). Examples of digital mapping include government survey plans which are now often available as 2D and 3D electronic data files. Apart from being in a more flexible form than paper maps, these are often used as a backdrop for geographical information systems (GIS). These can be thought of as spatial data bases where attributes, such as postal codes or land use, are attached to different parcels of land. The subject of GIS is large and beyond the scope of this paper.

A DTM may be thought of as a map with a third dimension - in other words, one which has been draped over a skeletal model which represents the shape of the same piece of ground. Thus, not only does one have the detail of what a point on the map represents but also the position of that point in space. However, while this concept may help one to visualise a DTM, the analogy is slightly misleading as maps are necessarily to scale but a DTM is scaleless, each point on its surface being represented by absolute coordinates. Only when plotting a plan derived from the DTM does the question of scale arise.

In summary a DTM is a full size replication of the area of ground under consideration held digitally in a computer memory.

Two main features of a DTM should be:

• that it can be easily created from the data supplied, and,

• that it can be edited and manipulated easily by non-expert users to produce the desired results.

Any DTM is only as good as the data supplied and it should be remembered that this data is really a sampling of the real surface at determined points. This sample must therefore be representative - for example it is no use picking up where the banks of a drainage ditch are (fine for a map) without determining how deep it is. To obtain an accurate idea of the shape of the ground one must remember to work in three dimensions all the time.

The method of construction of the DTM is also very important. Several methods exist but the most reliable and accurate is generally held to be triangulation, wherein data points form the nodes of triangles, the sides of which connect adjacent points. A triangle is the lowest order shape that can represent a three dimensional surface - it can help to visualise this by considering why tripods are used to support instruments, drilling rigs etc. It is not appropriate to consider the different mathematical methods of triangulation here, but the algorithms used should ensure that the triangles are the best available fit and allow each point to move independently of its neighbours so that only the area immediately surrounding the point is affected by any modification of its value.

The following is a description of some of the possible applications for sugar estates.

General The most obvious application is the production of three dimensional models to represent the original ground, any proposed changes to that ground by way of designed input and a record of the day to day changes as they occur. The data for these models can be collected from a variety of sources, including existing maps, aerial photography and land survey, all of which can be combined in the same model. The model can be modified with new data as time progresses and the model archived at intervals to obtain an historical record of progress.

The advantages of working with a DTM include being able to add data seamlessly and that the theoretical area of the model is without limit and, being scaleless, the whole area of interest can be included without prejudicing the ability to zoom in for a closer look. The larger capacity of DTMs now available, and ever increasing power of personal computers (PCs) mean that an incredible amount of information can be stored in one model.

Infrastructure Once a base model of the area of the estate has been wholly, or partially, formed, it is possible to design such items as feeder roads and drainage ditches. This is easily done by creating separate design models, either using the inbuilt facilities or external CAD systems, and then combining them with the base model to form the finished product.

A typical method would be defining the route and vertical alignment using the base model as a reference and applying a template along that route to achieve the correct shape - for instance a road embankment. The design model can then be compared to the original model to determine the quantities of material required and later compared to an 'as built' survey to monitor the accuracy of construction, which might be critical for drainage.

Field graded by laser control

Grading fields A similar exercise can be carried out but the design in this case would be a plane with the desired slope. Any drainage or irrigation furrows could be incorporated as modifications to the plane. As an added bonus, the model can also provide information to assist with the setting out of rotating lasers to control bulldozers and graders.

A facility within many DTMs which can be useful in this application is the generation of isopachytes - lines of constant height difference. These are obtained by comparing two models - for example those for the original ground and proposed field - and give a rapid indication of the amount of material to be moved, greatly assisting in the process of optimisation. An isopachyte with the value zero represents the cut/fill boundary.

Categorising by surface codes Most good DTMs include the ability to assign surface codes to different areas of the model. Some suggestions are given below:

• Height - areas within different height ranges can be identified.

• Slope - areas having slopes within certain limits can be identified as having the best prospects for growing cane with minimal grading.

• Aspect - areas which face in a general direction can automatically be given a surface code.

• Varieties - different crop varieties might have their own surface codes.

• Yields - codes could be set up for differing levels of yield and easily displayed. These could then be compared with some or all of the above to arrive at a good combination of factors.

• Land tenure - categories such as estate owned, small or large outgrower owned, estate leased, and settlement scheme can be given codes which are then applied to the area under tenure. It is then easy to calculate the plan and slope areas of different categories or of individual properties.

Drainage With triangulated models, the line of greatest slope for each triangle can be calculated and shown as an arrow. A display of all of these gives a very good indication of the drainage pattern of an area.

Irrigation Contours at any interval can be generated and these can help determine the line of rows to assist with erosion control. The coverage of centre pivot irrigation systems can also be easily displayed and an indication of the suitability of the land for these systems given.

Reporting A progress report to head office no longer needs to be a relatively sterile piece of paper. A continuously updated DTM already contains much of the information required and can be readily transmitted by electronic mail.

Training With the help of some inexpensive third party software, it is possible to set up tutorials for local staff, not just in the use of DTMs but also for explaining future developments.

3D views These can be invaluable for obtaining an appreciation of an estate, particularly for lay people and can ease the process of planning and obtaining any necessary funding or consents. It is possible to exaggerate the third dimension when looking at relatively flat areas to help gauge slopes and drainage. The model can be projected onto a zero datum and the third dimension then set to represent yield in different areas, for example. This might lead to some interesting conclusions when viewed in 3D. A 3D view of future development might lead to greater understanding and appreciation by staff. Some systems have the ability to create and play back 'fly-bys' or 'fly-throughs' of a model which can be both useful and impressive.

GUYSUCO has about 45 000 ha of land under cane distributed over eight different estates. Most of the land growing cane land is below sea level and some requires pumped drainage. The maintenance of hundreds of kilometres of drains leading to the pump stations is essential to maintain cane production. By the early 1990s maintenance had deteriorated and rehabilitation of drains was identified as a priority. Surveying capacity and equipment was upgraded so that more drains could be surveyed and correct invert levels restored to ensure good flow rates and high pumping efficiency. A digital mapping system was selected.

Typical digital mapping application

Many of GUYSUCO's maps are physically old and/or outdated. When possible, they are being digitised. Past work is then preserved and results of re-surveys can then be rapidly edited onto the maps. Digital base maps are also being used in conjunction with AutoCAD overlays of building designs for new civil works.

Future use will be to map new development areas of up to 5 000 ha. These maps will include drainage and road layouts.

The first GUYSUCO equipment, purchased in 1991, is still in use and a second set is planned to cope with increasing survey work and to improve survey services to the two regions of the industry.

SCJ has about 13 800 ha of estate cane, of which 8 800 ha is grown under irrigation at its Monymusk and Bernard Lodge Divisions. SCJ plans to redevelop 4 800 ha of its irrigation, mainly to long furrow with some centre pivot and drip trial areas.

The main reason for purchase of digital mapping equipment was to ensure accurate survey and land levelling for the long furrow. For centre pivots accurate survey is necessary for planning the machine sites and assessing drainage needs/requirements.

As with GUYSUCO, the second reason was to update mapping of the entire estate and obtain reliable field areas. Topographic mapping of all features, field edges, canals, drains, roads, houses and power lines is being done.

As expected, digital mapping is proving ideal for providing basemaps for detailed irrigation designs. Once mapping is complete the results are imported into AutoCAD where an overlay showing the irrigation design, canals, pipelines, etc is constructed.

Digitised cadastral maps are available from the government survey department and it is planned to use these as the base map for the for the detailed field by field topographic information obtained by the SCJ digital mapping. This can be done by controlling each detailed survey by a co-ordinate from a GPS receiver.

It has often proved difficult to estimate the area of cane belonging to private farmers and yet SCJ receives about 40 percent of its cane from this source and planning for it is important. Future use of digital mapping at SCJ might include area surveys of private cane fields.

MSC has about 3 400 ha of cane on its estate but receives most of its cane from 41 000 ha owned by private farmers. The average plot size is only about 1.0 ha and so management of cane deliveries and payments is a complex task. Many of the inputs that the farmer receives on credit are charged on the basis of the plot area and so it is essential to know this accurately. It is also important to have accurate plot areas to monitor yield trends and to detect possible fraudulent practice with cane deliveries.

Part of the Busia mapping project

All plots were surveyed manually when they were first developed. The industry is more than twenty years old and most plots have been replanted three or four times. Traditionally, the company relied upon the original survey for plot area and only resurveyed when a change of owner or plot subdivision occurred. This resulted in only about one third of annual replanting (about 7 000 ha) being resurveyed. The digital mapping system was commissioned in 1995 with the objective of speeding up the survey process and allowing a much greater proportion of replanting to be resurveyed. Since purchase, all replanted plots have been resurveyed although the annual total has only been about 3 000 - 4 000 ha per year due to high ratoon productivity. MSC has one set of equipment and it will probably require a second when normal replanting resumes.

As with other estates, there is a need to update past mapping at MSC, particularly of its nucleus estate where drainage improvements have been identified.

MSC is associated with another company, Busia Sugar Company Ltd. This will be a factory without an estate, relying entirely on outgrowers. Digital mapping was used to present to the government of Kenya a plan of the roads required to fully service the Busia region. Main features of existing cadastral mapping were digitised and road upgrades or new roads were highlighted.

Kinyara Sugar Works Ltd, Uganda (KSWL)

The KSWL estate has been redeveloped since 1993, after being abandoned in the mid 1980s during civil war. At present, the estate area is 6 800 ha and there are also 1 400 ha of outgrowers. All development has been mapped digitally.

Digital mapping has proved very useful in planning the overall layout of the estate fields, roads, and waterways for erosion control. At the field level, the contour generation facility enables appropriate row lengths and furrow directions to be selected.

A first survey for design work is done after land clearing and a second cane area survey is done after setting out and ploughing are complete. KSWL has seen the benefit of easy map editing. For example field areas changed with delayed clearing of debris from field edges; an airstrip dramatically changed one field.

KSWL has produced a base map covering the developed area. The base map has been compiled from "as made" surveys of cane fields. The map has been extended to cover the outgrower boundaries by digitising detail from existing government mapping at 1:50,000 scale. The whole area is controlled by the Uganda government grid system and this enables each outgrower plot to be located as it is developed. A discrete plot survey is done and two readings with a hand held GPS unit control the survey and enable it to be added to the base map.

This base map has been used to produce a map of the road development strategy for the region. This can be edited as various roads are completed.

This is an irrigated estate of about 10 000 ha, which replants up to 1 500 ha per year. The irrigation method may be sprinkler, long furrow or, increasingly, drip. Regardless of method, fields are mapped digitally at replanting and contours assessed to restablish row direction, slope and length appropriate for the irrigation or drainage needed. The required levels on a 30 m grid are compared to those existing, and cut and fill using graders is controlled manually in the field.

Recently, a conversion from sprinkler to drip irrigation for 1 200 ha has been initiated. New aerial photography has been digitised and the maps produced have been used as the base maps for the drip design.

This is an irrigated estate, also of about 10 000 ha, which has been redeveloping land (including bringing in some new land) at a rate of about 1 800 ha per year since 1992. The irrigation system is long furrow. The main reason for purchase of digital surveying equipment was to work in conjunction with laser controlled land levelling equipment to improve irrigation.

Fields are mapped after ploughing and various planes or combinations of planes are tested on the contour map to provide the right row length, and slope with minimum soil volume to move. This may involve introducing earth feeder canals into fields. For existing fields no more than 250 m3 /ha should be moved. Scrapers equipped with laser receivers are then used to level according to the desired plane. Fieds are remapped 'as complete'. For new land, excess volume is initially rough levelled by bulldozer with manual survey control.

GUYSUCO, SCJ, MSC and KSWL use LSS digital modelling software. All have the 'Elite' version; 'Plus', with a one million point capacity, may become necessary as skills develop. All systems were MS-DOS based; conversion to WINDOWS is under way.

Prior to the purchase of software, all estates were using manual field survey and drawing methods and so an electronic distance measurement (EDM) total station, personal computer and other hardware were also necessary.A basic set of equipment comprises:

Of the four total stations two have on-board datacards and two have connection to a Psion datalogger. The latter have given problems, mainly physical cabling and electrical contacts, and are not recommended. Minimum computer specification for the Windows version is easily met with standard machines. Original machines are still in use and these have only a 486dx 33 mhz processor, 8mb RAM and 100mb hard disk. The printer is needed for reports and an inkjet plotter allows the full range of surface colour codes to be printed. The minimum size plotter is A1 with A0 preferable (at œ1 000 more). Last but not least, a UPS is essential for a sugar estate.

RSSC and ZS have similar hardware but use software called SURPAC which is supported locally from South Africa.

Under serious consideration for the future is a full GPS receiver system instead of the total station. This is particularly applicable for large areas of new development which may need the survey and base map to be produced quickly. The cost is dependent on the accuracy required and could be double that of the basic total station system.

If proper training is given, digital mapping can be easily adopted by staff typically already employed by a sugar estate. This is the case at all the estates mentioned. The first LSS system was initially used by a Booker Tate Limited surveyor who had a few days preliminary training in the UK. The knowledge gained was an important factor in deciding to standardise LSS at the four estates as he became available to train at other locations. Similarly SURPAC was chosen for RSSC and ZS partly because it could be readily supported.

Digital mapping and modelling have a wide range of applications in agriculture, a range which is continually expanding as experience is gained of this fledgling technology. The increase in power of personal computers PCs, the falling prices of hardware and the availability of commercial software mean that its benefits need no longer be confined to a head office but can be viably disseminated to the field.

BTL has seen all the advantages of digital mapping in practice, namely:

• the speed, accuracy and reliability at which maps can be produced,

• the ability to edit mapping as physical change occurs,

• the quality of presentation,

• the ability to design and see the effect of changes, and

• the usability of the end product in other applications.

However one must not overlook the practical limitation of input data accuracy; for example survey staff may not be consistent in delineating features. Experience has shown that it is only the hardware of a digital mapping system that is likely to give problems in a sugar estate environment and these can be overcome with proper planning and back-up.