New Topo50 and Topo250 Maps for New Zealand

If you stay informed about maps, you may already know that Land Information New Zealand (LINZ) will be officially changing New Zealand’s Mapping System in September 2009. There will be several obvious changes for people who use LINZ maps for navigation, one of which is that New Zealand’s map grid will change. This is a consequence of the Geodetic Datum (from which latitudes and longitudes are derived and on which the entire maps are based) having been changed several years ago, and LINZ is finally updating its maps to catch up with its techniques. When this happens, LINZ also intends to make a collection of other substantial changes to how maps are produced, how they look, and how they’ll be used. Most obviously for people into tramping, the well known NZMS260 series (1:50000 scale) and the NZMS262 series (1:250000 scale) of maps will be completely withdrawn from publication, and respectively replaced by two new series’ of maps called Topo50 and Topo250.

LINZ has a large section of their website which describes the project in detail, explaining how it will affect people’s use of maps, GPS devices and other related systems. It’s good to see this actually happening — we’ve been hearing about it for several years now.

Everything you need to know is behind the link above, but I thought I might try to summarise the changes, perhaps to help people understand it better but as much to help myself to get a grasp on what’s changing and why. I’m doing my best to get this as correct as I can, but please keep in mind that I’m an amateur at this. If you notice inaccuracies or omissions, I’d appreciate it if you could point them out by posting a comment. If necessary, I’ll correct the post and credit as appropriate. Meanwhile if you’re keen to get more authoritative information from the source, a good place to start is the LINZ Publications & Other Resources page. The downloadable Topo50 map Reading Guide and the Where in the World Are We? booklets are especially helpful, and much of what I’ve written here is really just a dumbed down version of them.

For a long time now, LINZ has produced both the NZMS260 and NZMS262 series’ of maps, the former of which is used extensively for tramping. These maps are about as official as it’s possible to get for topographic maps of New Zealand. It’s taken decades to produce the entire series, with each map having been manually drawn. As the series of maps has been produced over such a long time, some maps don’t even match properly with neighbouring maps in properties such as colouring, especially if the maps were produced at different times. A few years ago, LINZ announced that as a major project, the map system would be changing and the various coordinate systems associated with New Zealand mapping would change with it.

There are a variety of reasons why it makes sense for LINZ to update the mapping system, but the most important is to make it work more nicely with the alternative “Geodetic Datum” that LINZ decided to adopt a few years ago. To understand why on earth this was necessary at all, let alone what a “Geodetic Datum” actually is and why LINZ cares about it, it’s necessary to understand something about how maps work and how they’re put together.

It’s all about projections

If you’ve dealt with maps before, you’ve probably heard something about map projections. A “projection” is the necessarily imperfect answer to the problem of how to represent the curved surface on a flat piece of paper.

Each of the maps in the NZMS260 series, the NZMS262 series, and probably most other series’ that LINZ publishes, is drawn on a flat sheet of paper, but the land that it represents isn’t flat. What it comes down to is that a flat map is not a perfect representation of the land it represents, and it can’t be. If every map in the series were laid side by side, the combined land area would look very skewed, and if the maps were pushed around to make the land appear closer to the correct shape, they would no longer line up properly side by side, and all of the nice, parallel lines of the overlaid map grid would no longer be parallel. In practice, the Earth is so large that the area represented by a single map is almost flat, and for most common uses (such as trampers taking compass bearings), it’s easily good enough for the job.

This is what a projection is. To ensure that the map can be displayed most usefully in a flat context, LINZ has to skew the shape of the entire country both so it’s not quite correct, and so it’s as un-obvious as possible that it’s not quite correct.

Points of reference and coordinates

Until recently, I’d intuitively assumed that things like latitude and longitude were very easy concepts. ie. With a specific coordinate, just draw a line from the centre of the Earth in whatever direction some brilliant maths indicates, and the geographic point for a given latitude and longitude will be where that line intersects the Earth’s surface. The intuition falls apart once it’s realise that to do this, just for starters, some way is needed for deciding where the centre of the Earth is. It falls apart even further when it’s realised that the Earth isn’t a perfect sphere, or even a sphere with mountains and valleys messing up the surface. It’s actually an oblique spheroid, which means it’s a flattened sphere that’s much wider in the middle than at the poles. The simplicity of projecting lines from the centre is getting less simple all the time.

You don’t necessarily need to know the centre of the Earth to draw a decent map that’s a good representation of an area, but you need reliable points of reference for which you do know the latitudes and longitudes already. The reference points can’t move, because if they do then it means you can no longer be certain about every measurement you’ve taken from them. New Zealand is a geologically active place, where the land moves around, and this is essentially the problem with the old NZGD1949 Geodetic Datum on which the existing LINZ topo maps have been based until now.

Geodetic datums define the reference system

This is where we come back to geodetic datums, because a datum is essentially the reference system on which everything is based. It might be something derived from the centre of the Earth, or it might be something else. It’s essential that it’s reliable and well understood, however, or everything derived from it becomes confused.

NZGD1949 is what’s known as a “local horizontal datum”, and also a “static datum”. It’s a reference system based on the known positions of trig stations around New Zealand that have been measured very accurately, and it also assumes that they never move. This isn’t exactly true, and the way the datum works makes it more difficult for surveyors to model geological movement without a lot of effort. Therefore this kind of system is not sufficient for use in New Zealand for some of today’s needs, because it’s not a reliable reference to the level of accuracy those needs require. Another issue with NZGD1949 is that having been designed to only fit New Zealand’s land-shape and nothing else, it’s not naturally compatible with much of the globally-aware navigation technology available today, notably GPS systems. To work with NZGD1949 and systems based upon it such as the New Zealand Map Grid, such technology has to incorporate lots of conversions. Through doing so, they waste resources and lose accuracy.

NZGD2000, the new Geodetic Datum which LINZ internally adopted in 1998 (but didn’t immediately use publicly in place of the NZMS260 or NZMS262 series’ of maps), is known as a “geocentric three dimensional datum”, and a “semi-dynamic datum”. It’s a referencing system based on a particular definition of the centre of the Earth and it’s been designed from knowledge that takes the whole planet into account. It’s designed around a model that estimates the shape of the entire Earth well beyond just New Zealand. Obviously the use of NZGD2000 doesn’t prevent New Zealand’s land-mass from morphing by about 5 cm relative to itself each year, but through its design it does help the surveyors and geographers at LINZ keep track of what’s actually moving around in New Zealand much more accurately. Otherwise it’s like trying to measure a distance while someone’s pulling the carpet out from underneath you. Also, being a globally-defined datum, it’s much more easily compatible with the GPS network, which operates in that context.

Collateral damage to coordinate systems

Being naive about the topic as I am, it hadn’t occurred to me until I learned of these changes is that latitudes and longitudes, which I’d always assumed were absolute everywhere, are really only subjective to the system used to plot them and that the systems vary in different places. When LINZ switches its maps to use NZGD2000, all places in New Zealand will be assigned a new latitude and longitude, as far as the predominant series of maps are concerned. Effectively, if you were to go to a specific position of latitude and longitude according to an old map and then again according to a new map, it will be as if New Zealand has shifted about 190 metres north and 10 metres east. (Check out the Differences section of the LINZ FAQ for more information about this.) What’s really happening, of course, is that those latitude and longitude lines are slightly offset from where they used to be because New Zealand is properly aligning itself with the same reference point used by much of the rest of the world.

The New Zealand Map Grid (NZMG) will also disappear, to be replaced by a new grid called the New Zealand Transverse Mercator projection (NZTM2000). This happens for the same reason, because the old NZMG was projected onto New Zealand using systems derived from the old NZGD1949 reference system. With NZTM2000, drawing the old map grid doesn’t quite work any more.

Even though LINZ has been doing its surveying using the new NZTM2000 for more than a decade now, it’s certainly possible to hack together maps with the old grids on them and use the old numbers and draw the old lines. In fact, this is what’s been happening over the past decade. During all this time LINZ has still released newly surveyed editions of their NZMS260 and NZMS262 topo maps with all the internally surveyed new coordinates converted back to the old coordinates for publication. This has been so that the new maps would be compatible with those already in circulation, giving LINZ more time to prepare for the change-over which is now here. Ultimately though, this is inefficient and leads to more and more complications and maintenance problems. Sooner or later, it all has to change.

Change happens on 29th 23rd September 2009

On 29th September 2009 (edit: 23rd September 2009), LINZ will finally release its new series of maps into shops. The old NZMS260 and NZMS262 series’ of maps are no longer being produced, and they’ll be completely withdrawn on that date. At the same time, the complete series of all Topo50 and Topo250 maps will be released, and LINZ is taking the opportunity to make several additional changes to the production system of its maps. Notably,

  • The standard printed size of map sheets will become smaller and more consistent. In the NZMS260 and NZMS262 series’, maps were printed on a non-standard, and very large, sized sheet. Some maps were even sized inconsistently with their neighbouring maps, especially if a small segment of land existed off to the side, and making an existing map slightly wider would remove the need to print a new map of mostly water. With the new Topo50 and Topo250 map series’, all map sheets will be produced as metric A1 size. In cases where a map will only have tiny amount of land to be nearly useless for land navigation, LINZ will overlap the maps with neighbouring maps to ensure that every map displays a useful amount of land. Therefore some land may be duplicated on multiple maps, but will also do away with maps that are 95% water.
  • Maps will be cheaper. LINZ really wants to encourage people to switch to the new Topo50 and Topo250 maps as soon as possible. Part of this encouragement is to make them available to anyone at the wholesale price of $3.50, as long as 20 or more maps are purchased. (ie. A minimum of $70 spent.) This means you could potentially replace your entire map collection relatively cheaply, or alternatively you could put together an order with several friends. LINZ is switching away from sub-contracting the printing of maps, and will now print them in-house, and this is contributing towards the cheaper costs (as I understand it, at least).

    To help people figure out which maps they might need, LINZ has provided material that compares the NZMS260 and Topo50 Map series’, including diagrams showing how grids of the map sheets overlap each other. If you have a collection of NZMS260 maps and need to know which Topo50 maps to buy for the same areas, it’s a good place to start.

The Topo50 maps are already accessible, in a sense. The complete database from which they’re produced is available using NZTopoOnline, part of the LINZ website which can be used to generate and print maps of sections of New Zealand on the fly. Presently the maps from NZTopoOnline don’t come out quite the same, however, because it’s an automated system that only has raw mapping data to generate from. Apart from having been printed professionally and with predictable consistency, printed LINZ maps, once available, will all be rendered more nicely as a consequence of people having gone through the data to ensure it looks readable, notably by doing things such as ensuring labels of features don’t overlap each other, and that kind of thing. With the natural move towards a more digital and centralised storage of all the information, however, it may become much more feasible in the future to print maps on demand, or have agents do so on LINZ’s behalf rather than require retailers to stock hundreds of different maps just in case someone might walk in wanting one. This is more towards the future, however.

Consequences for old maps

Obviously the old NZMS260 maps will still work, and it’s not as if geography changes simply because the New Zealand government decides to draw a new set of lines over it. There’s an underlying theme of encouraging people to stop using their old NZMG maps, but it might take a while for them to disappear completely. I certainly won’t throw out my maps simply because they’re old, and I doubt many of my friends will. Old maps are a great source of information about things like old tracks and routes that are no longer officially marked. Some people have lots of lines and marked up information drawn over their existing maps which is great for reference material when visiting somewhere, and that won’t be thrown out quickly. What I expect to do, however, is to move towards the Topo50 series of maps and take them out tramping with me.

The real problems will occur when communicating information about maps using two different systems, however. For instance, the potential of confusion if someone gives an NZMG grid reference from a 260 series map, which another person tries to apply to the NZTM2000 grid of a Topo250 map. New Zealand’s Land Search and Rescue is taking this seriously, and at the change-over date, all of SAR will immediately switch to the new mapping system in an attempt to avoid confusion when communicating between each other.

The confusion between the two map grids has been of particular concern, and a decision made to reduce confusion has been to design the new NZTM2000 grid so that the New Zealand land mass which it describes does not even overlap the same land mass in the NZMG when the reference numbers are the same. In other words, a grid reference in either system for anywhere in New Zealand will appear as if it’s pointing to open sea if it’s applied back to the wrong system.

Now it’s just a case of waiting

Hopefully this is a reasonable summary of not just what’s changing, but why it’s changing. The “what” part is easy, but having written this up I now feel as if I’m getting a better grasp on what the reasons and problems are. As I wrote earlier, I’d appreciate feedback and corrections, and whatever further thoughts or comments anyone might have on this.

I guess the most important thing to keep in mind is that it’s coming very soon. It seems like a very significant and important change in the use of maps in New Zealand, yet to date I haven’t seen much publicity of it outside tramping circles. I’ve wondered if there may be large clusters of people whom it might affect, who aren’t even aware that the entire mapping system is about to change.

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