The creation of a Survey Accurate Cadastral Map for surveyed areas in Trinidad & Tobago

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1 The creation of a Survey Accurate Cadastral Map for surveyed areas in Trinidad & Tobago Anesh Gopee, Keith Miller and Charisse Griffith Charles University of the West Indies Trinidad aneshtt@yahoo.com Abstract This paper advocates the establishment and use of a survey-accurate cadastral map (SACM) to depict surveyed areas in Trinidad and Tobago. An SACM can be simply described as a digital cadastral map that is constructed or generated from survey measurements of parcel boundaries, where the accuracy of each measurement has been maintained or preserved to a relatively large degree. The production of a series of such maps would contribute significantly to the spatial data infrastructure which supports and sustains the national cadastral system. This contribution could in turn, facilitate the move towards a multipurpose cadastre. The basic structure of a methodology which can be used for SACM production in Trinidad and Tobago is provided. This particular methodology adopts a least squares adjustment approach and involves the integration of the individual cadastral surveys of land parcels, into a unified coordinated framework. This framework can then be graphically presented in the form of an SACM. The proposed methodology is applicable to any area or district where there is an availability of cadastral survey data. SACM boundary marker coordinates are generated along with estimates of positional uncertainty for these coordinates. Certain procedures are used within the methodology to refine these estimates so that they provide a proper measure of the quality of the SACM coordinates. The methodology also possesses the ability to dictate the level of accuracy associated with the generated SACM. This is made possible through the use of GIS techniques to optimize the proximity of control points to the boundary markers to be coordinated. Major issues affecting the cadastral system of Trinidad and Tobago Cadastral plans are produced from cadastral surveys and contain detailed information regarding the locations and dimensions of land parcels (Dale 1976). The plans fulfill many important functions and form the basis of the integrity of the cadastral system of Trinidad and Tobago. However, a major issue that affects the national cadastral system is one that involves the isolated or insular nature of the existing plan data. This stems mainly from the fact that the majority of cadastral surveys that are undertaken are not connected to the national geodetic network. These particular surveys remain uncoordinated and are commonly referred to as floating surveys. Consequently, a large proportion of the national inventory of cadastral plans cannot be adequately tied or related to one another. The Lands & Surveys Division (L&SD) of Trinidad and Tobago is responsible for the acquisition, storage, maintenance and dissemination of all cadastral data for the country. The lack of an appropriate means to join or connect the individual or isolated plans severely compromises the ability of the L&SD to effectively represent the entire

2 collection of cadastral plan data as a unified cadastral database. The development of such a database (which would comprise of comprehensive or complete cadastral maps) can significantly enhance the national spatial data infrastructure (SDI). This would be very beneficial since the SDI provides the bedrock for the efficient and effective management of the cadastral system (Opadeyi 2002). A series of digital parcel index maps has been established by the L&SD for the entire country. These are the closest entities to comprehensive cadastral maps that exist in Trinidad and Tobago. It should be noted however that the parcel index map was originally developed primarily for the indexing of cadastral plans and as a consequence, does not possess the characteristics typically associated with an actual cadastral map. For instance, the parcel index map usually contains gaps or areas devoid of cadastral information. In addition, the map is not up-to-date and its spatial accuracy is low and even questionable at times (Robertson 1988). Another issue that affects the cadastral system of Trinidad and Tobago is the fact that the entire inventory of cadastral plans is maintained in a predominantly analogue or hard-copy state. The reform and upgrade of the national cadastral system is adversely affected by this particular situation, since the absence of digital plan data hinders the automation and computerization of a number of cadastral processes and operations. What exactly is a Survey Accurate Cadastral Map (SACM)? A survey-accurate cadastral map (SACM) is a computer-based map of all the land parcels in a particular area, compiled from survey-accurate measurements (Enemark et al. 1999). In some countries such as Australia, the SACM is commonly referred to as a survey-accurate Digital Cadastral Database (DCDB) (Williamson & Enemark 1996). A relatively high level of spatial accuracy is associated with the SACM, and this is one of the chief characteristics that defines such a map. This follows directly from the fact that the accuracy of the original survey measurements are maintained or preserved as best as possible during the compilation of the SACM. Additionally, the SACM is defined by a high level of comprehensiveness since it shows the entire land parcel framework in a particular area. The SACM also provides a consistent and accurate set of coordinates for the boundary markers that demarcate the array of land parcels. This combination of accuracy and comprehensiveness of parcel data, along with the superimposition of a cohesive coordinate system, form the fundamental aspects of an SACM. The establishment of SACMs is not a novel concept since historically, such maps have been used extensively in cadastral systems throughout the world. It should be noted however that the present or modern-day incarnation of the SACM resides entirely in digital form, and therein lies the power and value of this particular version of the map. The benefits that are offered by digital data significantly increase the utility and applicability of the modern-day SACM. Digital maps possess many advantages over their analogue or hard-copy counterparts, especially in terms of the ease in which they can be stored, reproduced, edited and processed.

3 Potential benefits of an SACM The SACM is by nature, a graphical product, however it is shown below that its benefits manifest themselves not only in the area of cadastral mapping, but pervade through the cadastral surveying and land registration processes as well. Cadastral mapping The implementation of a series of SACMs to illustrate the cadastral fabric would improve the current state of cadastral mapping in Trinidad and Tobago. This is because the traditional practice of presenting cadastral data on individual or separate cadastral plans would be supplemented by the representation of such data in the form of an SACM. Each SACM would have been systematically compiled from these individual cadastral plans, and would reflect the current geometric configuration of all land parcels in a particular area. Improved cadastral mapping would better serve the various land managers and other important users of land information. Since it is an accurate digital representation of the land parcel framework, the SACM could form the basis of computerized land information systems (LISs), and allow for detailed analyses to be undertaken at the parcel level. Such analyses have been relatively difficult to achieve in Trinidad and Tobago, due to the absence of a spatially accurate and complete parcel map. Cadastral data expressed in the form of an SACM, can be easily integrated with a variety of other spatial or geographic datasets. This is because the mere existence of coordinates (based on a common coordinate system) precludes the requirement for these other datasets to be directly referenced to boundary lines within the cadastral framework. In addition, the relatively high spatial accuracy of SACM coordinates would allow for the proper registration or spatial agreement between the different datasets and the cadastral layer. Thus, it can be seen that the adoption of the concept of an SACM can help facilitate the move towards a multipurpose cadastre. The establishment of such a cadastre is the goal of most countries today (Elfick et al. 2005). Cadastral surveying Improvements in cadastral mapping through the implementation of a series of SACMs would have a positive influence on cadastral surveying activities. This is because such maps would significantly aid the cadastral surveyor in the planning and execution of field surveys. An SACM would show or indicate the geometric relationships between the parcel of interest that is to be surveyed or redefined, and all the surrounding parcels (not just a select few). This is made possible since the SACM would provide a uniform system of accurate coordinates for all the boundary markers in a particular area. It has already been established that the SACM has the potential to adequately supply the map component required for comprehensive land information systems (LISs). One such LIS could be suitably designed to provide important links to the cadastral survey history of land parcels. In other words, the collection of all previous cadastral plans that refer to past cadastral surveys could be attached as attribute data to the different land

4 parcels comprising the LIS. This would help to simplify the research work required to be undertaken by a land surveyor prior to executing the actual cadastral survey. In addition, the SACM would facilitate the use of satellite surveying techniques such as those involving the use of the Global Positioning System (GPS) for the performance of cadastral surveys. In this case, the SACM would provide the requisite framework for the referencing of GPS-derived coordinates of boundary markers. Land Registration The national cadastral system places a significant amount of value on the parcel information provided by cadastral plans. Hence, there is a requirement for proper mechanisms to be formulated and implemented for the quality control and assurance (QC/QA) of cadastral plan data submitted to the L&SD, during the process of land registration. The plan checking procedure currently employed by the L&SD is not rigid enough to detect plans that are of sub-standard quality. This is because the procedure basically consists of a check on the magnitude of the cadastral survey misclosure, in order to ensure its conformance to acceptable cadastral standards. It should be noted however that the SACM could be used to provide the necessary infrastructure to implement a rigorous and effective strategy to modify the current plan checking procedure. Depending on the manner in which it has been compiled, a very useful feature of the SACM can be its inherent ability to provide a quantitative measure of how well a new cadastral survey fits into the existing coordinated framework. This strategy could be used to verify the identity or existence of a parcel of land being registered, and to check the accuracy of the boundary information provided by each plan. Further benefits could be derived since this strategy could also provide certain aspects of the checking procedure the ability to become automated. In addition, the adoption of the SACM would significantly help to address the backlog of unchecked cadastral plans that presently exists, resulting in the undesirable delays that are typically associated with the land registration process. This is because apart from its scope for automation, the improved plan checking procedure mentioned above, would also reduce the need for checks to be undertaken in the field. The standard approach used to produce an SACM The shortcomings of the national cadastral system coupled with the potential benefits of an SACM provide suitable justification for the adoption of this type of map to represent surveyed areas in Trinidad and Tobago. Of all the countries that have recourse to some form of SACM, Australia is certainly ahead of the pack and can be considered a pioneer of the modern-day SACM (Williamson 1996). There exists an established method or approach for creating an SACM, which has been employed by many Australian states as well as a number of other countries. This approach has proven thus far to possess a high success rate. It basically involves the digitizing of existing cadastral maps and the incremental upgrade of their spatial accuracies over time, using accurate boundary dimensions obtained from

5 cadastral surveys (Williamson 1996). This incremental upgrade eventually creates the desired end-product which is essentially an SACM. If this exact approach is to be used in Trinidad and Tobago, the digital parcel index maps would have to be called upon and utilized for the creation of the required SACM. Unfortunately, problems would inevitably be encountered in attempting to upgrade this preliminary cadastral framework. This is primarily because the spatial accuracies of these index maps are of a low order and do not vary in any uniform manner. Hence, due to the inherent accuracy limitations of the index maps, this standard approach cannot be adopted for use in Trinidad and Tobago. An alternative approach to produce the SACM is required, one that is specifically adapted to the unique characteristics and requirements of the national cadastral system. Proposed methodology for SACM production in Trinidad & Tobago This paper suggests the use of cadastral plan data as the basic raw material for SACM production in Trinidad and Tobago. In addition, a mathematical treatment is proposed in order to georeference and merge the different sources of plan data. This particular treatment is based upon an application of least squares adjustment techniques. Least squares is the one method of estimation or adjustment most commonly used in surveying and mapping. Adjustment is required when there are redundant observations i.e. more measurements than are necessary for a unique solution of the mathematical model that defines the survey problem being investigated (Davis et al., 1981). In the context of SACM production, least squares provides a systematic way for discrepancies in measurements for the same boundary line to be reconciled, based on the accuracies of their respective cadastral surveys. Apart from adjusted coordinates, estimates of positional uncertainty are also provided as a useful by-product of the least squares adjustment. This is yet another advantage of adopting a least squares approach. The proposed methodology utilizes the measurements (bearings and distances) appearing on the existing cadastral plans as the inputs into the least squares adjustment process. These plans serve as records of the individual cadastral surveys of the particular area for which the SACM is to be created. The survey measurements are processed along with additional control data acquired in the field. This control data refers to the fixed points or constraints that are necessary for the propagation of coordinates throughout the network of land parcels. The individual or isolated cadastral surveys are then rigorously tied to one another so that a coordinated or georeferenced framework is established. The flow chart below summarizes the basic structure of the proposed methodology: Cadastral plan data (Bearings & Distances) + Control data (Constraints) Least Squares Adjustment SACM (Boundary marker coordinates & positional uncertainties)

6 Refinement of the positional uncertainty determined for SACM coordinates The proposed methodology is able to provide each of the boundary markers demarcating the various parcels of land depicted on the generated SACM, with two fundamental attributes. The first or primary attribute is a set of survey-accurate coordinates, whilst the second refers to the positional uncertainty associated with these coordinates. The correctness of the positional uncertainty (which is expressed as an error ellipse ) computed for boundary marker coordinates is a critical issue if a determination is to be effectively made of the quality of the georeferenced of coordinated cadastral framework. In order to address this particular issue, the methodology consists of a multi-tiered least squares adjustment process undertaken using a combination of observation equations as well as condition equations. Condition equations are used firstly to undertake a preliminary adjustment of each individual survey (independent of the wider network). A rescaling of the apriori covariance matrix of each set of survey measurements by a factor equal to the computed unit variance is then performed. In this way, a covariance matrix that properly reflects the quality of the survey measurements is systematically derived. Following this, an adjustment of the entire network of parcels is undertaken using observation equations. The survey measurements and their respective covariance matrices serve as inputs into this final network adjustment. Adjusted SACM boundary marker coordinates are generated along with their associated aposteriori covariance matrix. It is the view of this research that the dimensions of the error ellipses derived from this particular matrix provide a proper measure of the accuracy of the computed SACM coordinates. GIS-assisted least squares The proposed methodology also makes use of certain GIS techniques to dictate or control the level of accuracy associated with the final adjusted coordinates computed for boundary markers. This is achieved by inputting the preliminary cadastral network into a GIS environment such as that provided by the ArcView 3.2 software. An attribute value is then assigned to each boundary line that constitutes the network. This value is derived from the computed covariance matrix of the survey measurements pertaining to that particular line. It should be noted that the separation (along the network) between the control points and boundary markers is a major factor that affects the variances of the final adjusted coordinates for these markers. The proposed methodology uses the Network Analyst extension and its associated tools, to calculate the service areas of proposed control points, so that their proximity to the boundary markers could be optimized. By doing this, it can be ensured that the variances of the adjusted coordinates do not exceed a certain specified value. This maximum value is predetermined and is dependent on the level of accuracy that is expected to be associated with the generated SACM. Conclusion This paper has demonstrated that numerous benefits could be derived through the use of an SACM for the representation of the cadastral fabric of a particular area. A suitable

7 methodology for SACM production in Trinidad and Tobago was also outlined. An important advantage of this particular methodology is its ability to deliver coordinates of a specified accuracy. In addition, it provides properly-determined measures of the positional uncertainty of the SACM coordinates. Apart from being an accurate graphical representation, important numerical data could also be derived from the generated SACM. The production of the SACM is therefore an important operation that could be incorporated within the national spatial data infrastructure. The features and characteristics of the SACM are conducive to the establishment of a multipurpose cadastre and could be harnessed to effect positive changes to the cadastral system of Trinidad and Tobago. References Dale, P.F., 1976, Cadastral Surveys within the Commonwealth (London: HMSO) Davis, R.E., F.S. Foote, J.M. Anderson, and E.M. Mikhail, 1981, Surveying Theory and Practice (New York: McGraw-Hill, Inc.) Elfick, M., T. Hodson, and K. Wilkinson, 2005, Managing a Cadastral SDI Framework Built from Boundary Dimensions, FIG Working Week 2005, Cairo, Egypt, April 2005 Enemark, S., W. Effenberg, and I. P. Williamson, 1999, Framework for Discussion of Digital Spatial Data Flow within Cadastral Systems. The Australian Surveyor, 44(1), Opadeyi, J., 2002, Spatial Data Infrastructure and the Cadastral System of Trinidad and Tobago: the Caribbean Experience, FIG XXII International Congress, Washington D.C., USA, April 2002 Robertson, M., 1988, Land Information Management: Problems & Perceptions, Presented Papers, The University of the West Indies, St. Augustine Williamson, I.P. and S. Enemark, 1996, Understanding Cadastral Maps. The Australian Surveyor, 41(1), Williamson, I. P., 1996, Coordinated Cadastres A Key to Building Future GIS. Proceedings of the Regional Conference on Managing Geographic Information Systems for Success, Melbourne, July 1996, 60-69

A Geocoded Cadastral Fabric as a Precondition for a Sustainable Land Management System

A Geocoded Cadastral Fabric as a Precondition for a Sustainable Land Management System A Geocoded Fabric as a Precondition for a Sustainable Land Management System Gottfried KONECNY, Germany; J.P. LAUZON, Canada; Abdul Salam MOHAMMED, India Key words: SDI,, Parcel Boundaries, GPS-GNSS, Land