BRIEF HISTORY ON SURVEY MARKS Across the world survey marks were built to form a network in which surveyors can tie into, such that their survey is on a known system that most are familiar with. According to the Chief Directorate: National Geo-spatial Information there are approximately 24 000 town survey marks located in 122 cities which form the network. EThekwini has approximately 3 000 town survey marks that are being audited every year. Sadly this number is decreasing which are outlined in this article.

OVERVIEW The co-ordinate network developed in South Africa comprises of Trig beacons, Minor control, Bench marks and Town Survey Marks (TSMs). The importance of these control points can cost an enterprise millions of rands if there were any discrepancies in the survey due to inconsistencies with the network control. By Law the Surveying and Land Information Department at EThekwini Municipality has to ensure that the Town Survey Marks are maintained and are reliably for survey use. This article is based on the responsibilities and challenges we face whilst conducting this audit. Furthermore public awareness and structural developments will be discussed to ensure that our TSMs are protected.

Author: Merylene Naidoo Senior Survey Technician

Surveying and Land Information Department EThekwini Municipality

June 2016

History

Structural Components

Raising the cover

Future modifications

Locality Sketches

Purpose of the audit

Process of the audit

Extent of the audit

Public portal

STRUCTURAL COMPONENTS Town survey marks (TSMs) were designed to withstand daily vibrations from heavy motor vehicles, construction works within the area and any form of turbulences. The cast iron circular box measuring at a diameter of 240mm and a height of 200mm is placed on concreted platforms of a minimum height of 150mm. This allows the box and the concrete to absorb any sub-surface shifts that may occur without any disruptions to the co-ordinated brass peg which is also fixed in a concrete column that range to a height of 1m. The cover is also made out of cast iron and is secured with a chain bolted to the inside of the circular box. A tag is placed on the chain representing the TSM number.

FUTURE MODIFICATIONS The issue that we face is that the covers are being stolen or removed. The chain that secures the cover to the inside of the box gets rusted and broken off. Replacing these covers would cost approximately R300 each. Our department has run a few pilot tests to determine the best suitable solution to replace the existing ones. Some of the solutions are: 1. Cables to secure the cover to the box. 2. Locking covers with allan keys. 3. Currently we have contracted an individual to cast covers made out of polymer concrete. This is a more durably and cheaper replacement solution than the current cast iron covers.

RAISING THE COVER Many of the TSMs were placed at street intersections which would need to be raised as and when the road had been reconstructed or resurfaced. The design allowed for the circular box to be raised without disturbing the brass peg. Contractors are obligated to preserve these markings similar to boundary beacons. They may relocate the TSM or raise it and ensure that it is co-ordinated if moved. This however is not being done as contractors feel that it delays their work flow and is an added cost. Hence we lose so many TSMs.

PURPOSE OF THE AUDIT Town Survey Marks form one of the fundamental commencement points to any survey that needs to be on a known network. Surveyors can establish control for a 3 dimensional network and ensure the accuracies are according to survey standards. The heights were obtained by spirit levelling for millimetre accuracies to be achieved however 325 TSMs do not have heights which are located in Chatsworth, Pinetown, Westville, Durban and Tongaat. Trig beacons can also be used however the 3rd dimension (elevation) is to 0.1m. Many surveys require a higher accuracy than that. The TSMs are to 0.001m accuracy. Due to the high level of demand, the municipality has been instructed to maintain these control points by law, hence the formation of conducting an audit.

PROCESS OF THE AUDIT Town Survey Marks are located, cleaned and the brass studs are checked. Covers are replaced if necessary or alternatively filled with soil if located on tar surfaces (if the covers cannot be firmly attached). The TSMs are also painted with yellow paint for easy identification. Our database is then updated accordingly.

LOCALITY SKETCHES Most TSMs were accompanied with locality sketches to aid with locating the TSM at a later stage. With the advancement of technology a GPS can easily locate these marks. No more tape measurements are required. These sketches have the TSM number located at the top corner with a detailed sketch of dimensions and road names. Land marks are also illustrated on the sketch with a description of the TSM. Ideally our department would like to update these sketches however this is a time consuming process and most surveyors have the means to a GPS device to easily locate them. Google Earth can also be used to locate these TSMs.

The Chief Directorate: National Geospatial Information (NGI) is, in terms of the Land Survey Act (Act 8 of 1997), responsible for the establishment and maintenance of the South African national horizontal and vertical control survey networks. (Vorster and Koch 2013) In terms of Section 42(4)(a) states that, “A local authority shall within its area of jurisdiction be responsible for the maintenance to the satisfaction of the Chief Director, of any town survey marks erected in terms of this section,

and should the local authority fail to maintain any town survey mark the Chief Surveyor‐General may undertake any necessary work at the expense of that local authority.”

EXTENT OF AUDIT The EThekwini municipality extent is approximately 255594ha which ranges as far as Cato Ridge, Tongaat and Umkomaas (See left). The red rings indicate the areas in which the TSMs are located. Areas such as Pinetown, Westville, Chatsworth, Durban CBD and Tongaat made up the regions in which the TSMs were built. The black dots on the image above represent the TSMs.

EThekwini Municipality

Ugu Municipality

Umgungundlovu Municipality

Ilembe Municipality

SUB SECTIONS A closer look at the different regions we can establish the pattern and development of urban and suburbs within the municipality.

The inner and outer region comprises 566 TSMs ranging from Pinetown, New Germany, Westville, Newlands East, Tongaat and a portion of Durban central.

The south region comprises 900 TSMs within Chatsworth and a portion of Durban central. The north region includes 1336 TSMs in the remainder of Durban CBD extending towards Umhlanga. A total of 2802 TSMs situated within the EThekwini municipality which are currently being located and maintained by our team. This is no easy task therefore it is divided into 3 regions as outlined above i.e. the Inner & outer region, Southern region and Northern region. This number will be reduced at the end of the audit as many TSMs are covered by tar or destroyed.

PUBLIC PORTAL

As part of our initiative of ensuring that the public are well informed we will be creating a portal were co-ordinates of TSMs can be obtained in different formats (.shp, .csv, .kmz etc.) which can be extracted with supporting information to indicate the status of the TSM (covered by tar, demolished, disturbed). This will allow the public for easy access at any time, at any place and will decrease the necessity of visiting our counter for information. This will save you time and money. CONCLUSION The Land Surveying Branch has ensured the needs of the public are met by providing services that can help make their daily tasks trouble-free. This audit will be conducted annually to ensure consistency and reliability of TSMs.

REFERENCES Chief Directorate: National Geo-spatial Information, Trigonometrical beacons, town survey marks & bench marks, viewed 24 May 2016, from http://www.ngi.gov.za/index.pnp/what-we-do/geodetic-and-control-survey-services/36-trignometrical-beacons-town-survey-marks-bench-marks Vorster, P. and Koch, S., TrigNet, South Africa’s GNSS base station network: past, present and future. Land survey Act 8 of 1997

For any queries and information please contact our department: Phone: 031 311 7265 Email: Merylene.Naidoo@durban.gov.za