Skukuza Rail Bridge

[Kevin Wilson-Smith]

Dayle - having spent some time in Kruger i know exactly the bridge.

On my material there is no rail line shown - possibly because officially it is discontinued, although I know staff often still use it with hand trolleys.

However, I have provided a location for you accurate to within 200 metres either side, based on the fact that it is to the right of the guest outs, but just (just to left) of the Public Picnic areas. Maybe later in the year when I go to Kruger I could get it spot on for you.

[John Ashworth]

Kevin, have you got a GPS? You could get the coordinates next time you're there.

[Dayle Coombe]

Thanks Kevin that is perfect.

John
There is an site that has GPS co-ordinates for quite a few of the railway stations, our GPS system is a marine one, and not connected at the moment, otherwise I could have had great fun putting in all the station co-ordinates. http://gps.handitourist.co.za/

[Dayle Coombe]

This is the only map I could find that still showed the old line, I don't know when it was printed, it has no date on it, but the cost of the map book was R1,45, so I am guessing its quite old.

Sorry, meant to add that it seems that the bridge is where the line ends.

[John Ashworth]

Dayle, that's interesting about the marine coordinates for GPS.

Mike H had problems when he tried to use GPS to demarcate the boundaries of his property near Bela Bela a year or so ago and found that the South African surveying maps didn't use any of the systems that were on his GPS. Eventually, after an internet search and a lot of phone calls, he found someone who could give him the conversion formula.

My experience of using GPS is in Sudan where we just use latitude and longitude.

[Dayle Coombe]

This one we have gives you the underwater view as well, to be honest I don't really have a clue how it works, its got an echo sounder etc, its a bit on the big side though, not exactly a nice handy one to carry around.

[Kevin Wilson-Smith]

Dayle - I do have a GPS - the software I am using above is actually the GPS software!

The co-ordinate story can generally be fixed by going into the set-up.

John - your comments about the Mikes issues - I do not understand this! I have never had a problem in that respect - set the GPS to latitude and longitude, adjust for South Africa. GPS's do use other systems - do not use them - a waste of time! Everyone uses latitude and longitude - and because a GPS is a GPS I have not found one yet that does not have it!

Dayle - if you want me to help you with yours, I can maybe do so if you want.

[John Ashworth]

Kevin, get Mike to give you the details, but my understanding is that the GPS was indeed in latitude and longitude but that the surveyor's map was in some different system, hence the need for the conversion formula.

[Kevin Wilson-Smith]

John - OK I misunderstood - that makes sense.....

[Steve Appleton]

The problem is probably that some of the surveys and mapping were done before the adoption of the latest World Geodetic System - WGS84 coordinates as used by the GPS system. I quote from Wikipedia http://en.wikipedia.org/wiki/WGS84:

The new World Geodetic System was called WGS 84. It is currently the reference system being used by the Global Positioning System. It is geocentric and globally consistent within ±1 m. Current geodetic realizations of the geocentric reference system family International Terrestrial Reference System (ITRS) maintained by the IERS are geocentric, and internally consistent, at the few-cm level, while still being metre-level consistent with WGS 84.

The WGS 84 originally used the GRS 80 reference ellipsoid, but has undergone some minor refinements in later editions since its initial publication. Most of these refinements are important for high-precision orbital calculations for satellites but have little practical effect on typical topographical uses. The following table lists the primary ellipsoid parameters.

It goes over my head, but I am sure any other normal FOTR member would undersatnd it!

[Steve Appleton]

What a pity that the line through the Park was closed and lifted. Imagine the tourism potential it would have today.
Of course the track bed still exists through the Park and all it would take is a few odd lengths of steel rails with sleepers attached to rebuild it. Oh, and a span or two for the bridge at Crocodile Bridge!
I wasn't aware that after closure, the northern section was initially left as a branch to Skukuza. I thought the whole line was closed in one go.

[Mike Haslam]

Steve A is correct.

The boundaries of my property were provided by the deeds office in what is called Lo 29, not the usual lat/long.

Projections Used In South Africa

The Chief Directorate: Surveys and Mapping is mandated in terms of the Land Survey Act (Act 8 of 1997) to establish and maintain a national control survey network. All cadastral parcels and surveys, engineering surveys as well as most GIS projects are referenced to the national control survey network.

Numerous projections are used in South Africa especially for mapping. The official “issue“ coordinates of the national control survey network (and hence most surveys) are projected using the Gauss Conform Projection.

The Gauss Conform Projection, is the Transverse aspect of the Mercator projection (ellipsoidal form only, turned about 90 so that the projection is based on meridians and not the parallels). This projection is used for the computation of the plane yLo and xLo coordinates, commonly known as the “Lo coordinate system”

After much research and hunting, I found a friendly guy in the surveys dept who gave me a speadsheet to convert Lo29 to lat/long. After that, I could use my GPS to locate my boundry pegs. It worked fine.

[John Ashworth]

Thanks. Mike. I can't say I understood every word of it, but I got the general principle.

[Steve Appleton]

Some interesting concepts on the Ordinance Survey website. One is that a local reference system is necessary because, whilst WGS84 is defined globally, the relative locations of indivdual points move due to all sorts of reasons, not least of which is the movement of the tectonic plates and the geodetic centre of the earth. This causes your house to move constantly relative to WGS (as in GPS) latitude and longitude!!! I often wondered why things in my house go bump in the night; now I know!
This has big implications for the legal eagles because unless your property is marked out relative to a local, static reference point (eg Lo 29), the GPS coordinates for the corner pegs move constantly. That could lead to huge arguments with your neighbour when, say in 5000 years, he discovers that his boundary stream is now wholly within your own property!
To quote the OS site, which incidently has a more clear explanation than Wiki http://www.ordnancesurvey.co.uk/oswebsi ... uide4.html. And all this started out as a question about the location of a disused railway bridge! Are you still with us, Dayle?

There are a couple of points to note about this definition. Firstly, the ellipsoid is designed to best-fit the Geoid of the Earth as a whole. This means that it generally doesn't fit the Geoid in a particular country as well as the non-geocentric ellipsoid used for mapping that country. In Great Britain GRS80 lies about fifty metres below the Geoid and slopes from east to west relative to the Geoid, so the Geoid-ellipsoid separation is ten metres greater in the west than in the east. Our local mapping ellipsoid (the Airy 1830 ellipsoid) is a much better fit.

Secondly, note that the axes of the WGS84 Cartesian system and, hence, all lines of latitude and longitude in the WGS84 datum, are not stationary with respect to any particular country. Due to tectonic plate motion, different parts of the world move relative to each other with velocities of the order of ten centimetres per year. The International Reference Meridian and Pole and, hence, the WGS84 datum, are stationary with respect to the average of all these motions. But this means they are in motion relative to any particular region or country. In Great Britain all WGS84 latitudes and longitudes are changing at a constant rate of about 2.5 centimetres per year in a north-easterly direction. Over the course of a decade or so, this effect becomes noticeable in large-scale mapping. Some parts of the world (for example, Hawaii and Australia) are moving at up to one metre per decade relative to WGS84.

[Steve Appleton]

Hmm, the Flat Earth Society might have something to offer in terms of simplicity after all! Are they still taking members?