The Geoscience Raster 250k Maps: Which one?

This article deals with two frequent questions:

1. Which version of the NATMAP should I buy; Mapsheets or Mosaic?
2. Subdividing the Mosaic and Converting to OZF2 or, How do I extract a smaller section of the Mosaic, for example to use with OziCE?

and there are some other Resources; second opinions, and so on.

Which one?

The sections are below. You can skip straight to the Summary if you just want an answer without an explanation.

• What's the same?
• What's different, and why?
• Examples
• Summary

What's the same?

Firstly, both the Mapsheets and the Mosaic contain exactly the same information, the same detail, everything. They also cover exactly the same area, that is, all of Australia. The price is also the same. Both can be used with GPS software for trip planning and for moving-map systems. In the case of the Mosaic, you should check that the software supports the lat/long projection used. Two popular, Australian GPS software packages that fully support the Mosaic maps are:

1. Oziexplorer 3.95c or later
2. TrackRanger 3.2 or later. The forthcoming Version 4.0 will have additional features specifically to support the Mosaic map.

What's different, and why?

File size

The main difference is that the Mosaic is one, giant, map of Australia, with a filesize of about 700Mb. The Mapsheets are 513 individual, smaller maps, each of which is about 2-4Mb in size. The total size adds up to over 1.5Gb (1500Mb) because there is a slight overlap between maps, and because each map includes the legend (information about the map).

So the first consideration is; do you want one large map, or lots of smaller ones. The Mosaic is seamless; scroll anywhere in Australia and you never need to load another map. The Mapsheets aren't seamless; scroll far enough and you'll need to load a new Mapsheet, although all GPS software automates that process. It does mean that if you're at the edge of a map you can't "see where you're going" until you load the next map.

Although the Mosaic is a large file, you don't need a large amount of memory (RAM) to use it. That's because most software only loads what it needs at the time, not the whole map. That's even true of the Mapsheets, too.

Distances

Distances are handled differently. The Mapsheets are constant in every way; a given distance on the map always translates to a given distance in reality, and that distance is represented in exactly the same way on each map.

The Mosaic varies. Within a map, a given distance on the map equates to a different distance in reality depending on whether you are measuring north-south or east-west. And the ratio betwen the two changes depending on your latitude (how far away you are from the equator). That means you can't measure distance accurately on a printed version of the map with a ruler. However, some GPS software, like Oziexplorer (version 3.95.3c onwards) and TrackRanger (3.2 onwards), compensate for this and applies the necessary corrections so its distance measurements are accurate.

So why do the Mosaic's distances vary?

Because the earth is not flat. Maps are flat. Obvious, but that is the problem cartographers face, creating a flat representation of something that is spherical.

Think about peeling an orange, and trying to lay the peel flat. It'd work for a small section, but a bigger section simply wouldn't lay flat properly.

Now you can begin to see the cartography problem. The individual Mapsheets are only about 150km wide (east-west) by 110 deep (north-south). That equates to a little bit of an orange peel, which can be flattened quite easily into a map.

But all of Australia is bigger, and is a reasonable proportion of the earth's surface. In terms of the world's surface, it looks like this:

so we're trying to flatten a big piece of orange peel, and that causes a problem.

Because of the flat map/spherical earth problem, any given map of the earth will compromise one or more of the following:

1. Shape,
2. Distance,
3. Direction, or
4. Area,

and the larger the area, the greater the compromise, or distortion.

So the compromises for the Mosaic have to be different to the compromises for the Mapsheets, because the Mosaic covers a much, much larger area.

The technical difference between the Mapsheets and the Mosaic is the projection. A projection is the means of representing a spherical earth onto a flat map. The Mosaic map uses a lat/long projection. Lat/long is good for covering large areas, but the scale does change slightly. The scale is the distance on the map relative to the distance in real life.

The picture shows lat/long lines around the earth. The horizontal lines are lines of latitude (north-south), and the vertical are lines of longitude. As you can see, the lines of longitude get closer together the further away from the equator you go; in other words, they get closer together as you change latitude.

That means with lat/long projections the scale of the map changes as you move in relation to the equator. The compromise made has been Distance and Area. With the Mosaic, one pixel (point on a computer screen) equates to 36m in the northernmost part of Australia. In Tassie, in the south, a pixel is 26m. With the Mapsheets, a pixel is a constant 31.75m, everywhere.

This means if you print the Mosaic map out (or part of it), and start to measure distances with a ruler, you cannot simply add up the distances and expect them to be correct. You can with the Mapsheets.

To illustrate the point, the images below are of the maps in Tasmania.

Mosaic. Both blue lines are 10km long. The north-south line has been copied to lie east-west for comparison.	Mapsheet. 10km is the same map distance north-south and east west.
The Mapsheet image is from the Raster 2002, not 2003. This demonstrates the new standardised look with some of the older maps.

The scale of the Mosaic is not constant within the map, depending on whether you're heading north or south. It only becomes exactly constant on the equator. But the northernmost part of Australia is relatively close to the equator (about 1200km), and the difference is not very much:

Mosaic. Both blue lines are 10km long. The north-south line is marginally longer than the east-west, but it's barely noticeable at this latitude.	Mapsheet. 10km is the same map distance north-south and east west. Exactly the same as it was all the way down in Tassie.

Coordinates

Just because the Mosaic uses a lat/long projection doesn't mean to say you can't use UTM coordinates with it. Both the Mosaic and Mapsheets can use either set of coordinates.

Examples

Melbourne

The Mosaic looks different to the Mapsheets because of its different projection. The example below is the two maps running on a PDA using OziexplorerCE software (OziCE), showing part of metropolitan Melbourne. Both have been set so the same point is located in the top right-hand corner, yet they cover a different area.

Mapsheet	Mosaic
Note that although the base map data is the same, the location of the descriptive text varies.

The image below is of the Mosaic map. The magenta rectangle shows the limit of the OziCE Mosaic screenshot above, and the blue rectangle shows the OziCE Mapsheets screenshot.

(click the image for a larger image, which also shows a greater area. Click your browser's "Back" button to return.)

Cape York

This example is at Cape York, the northernmost point of Australia.

Mapsheet	Mosaic
Note that although the base map data is the same, the location of the descriptive text varies.

The image below is of the Mosaic map. The magenta rectangle shows the limit of the OziCE Mosaic screenshot above, and the blue rectangle shows the OziCE Mapsheets screenshot.

(click the image for a larger image, which also shows a greater area)

Cape York is a lot closer to the equator than Melbourne is, so the lines of longitude are a lot further apart. This has an effect on the amount of map we can see on the screen; it's wider relative to the more southern maps, but just as deep (the lines of longitude, east-west, are a constant distance apart irrespective of how far away from the equator you are). It is also interesting to note that the Mosaic is actually at a slightly smaller scale here than the Mapsheets, whose scale and distance remain constant all over Australia.

Summary

	Mosaic	Mapsheet
Size	Huge (about 700Mb)	Individual maps are small (about 2-4Mb, total about 1500Mb)
Distance measuring	Inaccurate (unless you use software)	Accurate, using software or a ruler.
Consistency of scale	Varies, different scales according to latitude	Constant north and south, and over all maps
Loss of functions	Some Oziexplorer functions may not work, as per Ozi's Mosaic Page which lists Print Map to Scale as an example. The same potential restrictions will be true of other software, as the issue is the projection, not the software. However, I've not found use of the Mosaic to be an issue.	None.
Coordinates	GPS software can use UTM or lat/long	GPS software can use UTM or lat/long
Seamless maps	Yes	No, have to load each one (moving-map software does this automagically)
Cost	Same as Mapsheet	Same as Mosaic. You can't save by buying indvidual Mapsheets.
Other maps included	A single map image of all 1:1,000,000 NATMAPs and a Landsat Satellite Image Mosaic of Australia	None
Suitable for moving-map	Yes	Yes

Notes

1. If you buy the Mosaic and you want a smaller map, you'll need to subdivide it. But that's not a difficult process, doesn't require any software you don't already have and doesn't need a powerful computer.

OK, so what's your recommendation then?

The whole purpose of this was for you to make up your own mind :-) But here's my recommendation for Australians using digital maps to travel; if you've got the space, go for the Mosaic, especially if you own an older Mapsheets version. You'll have one seamless map which you can subdivide into smaller sections which overlap as much as you like, so you don't need to "fall off" the end of one map before you get to another. And pretty much all you'll lose is ability to measure distances on a printed map, which I don't think is that much of a problem given there is software that can compensate for the differences. Finally, you get a 1:1,000,000 map and a satellite image too, so it's better value.

---

Subdividing the Mosaic and Converting to OZF2

For some people, the big problem with the Mosaic is the image size. For people who only use maps with notebooks or desktop PCs a few hundred megabytes is not normally a problem. For those who use the maps elsewhere, notably running Oziexplorer CE on PDAs, it's a bigger problem because these devices typically don't have enough storage capacity. This is how to export sections of the map, for example for use with OziCE:

What you need

1. The Mosaic map
2. The Raster Viewer, as supplied with the Raster 250k maps on the CDs, or ER Viewer, from http://www.ermapper.com/
3. Oziexplorer 3.95.c (this version is modified to work with the Mosaic maps). You could probably use the smaller maps with TrackRanger 3.2 onwards too, but I've not tested this.
4. Img2Ozf2 (if you wish to convert to OZF2)

How to do it

These instructions are an expanded and illustrated version of those on the Oziexplorer website.

1. Start Raster Viewer, and load the 250k Mosaic map.

2. Select the part of Australia you want. Suggest a fairly small part to begin with as a test, like your nearest town.

3. In the bottom-right corner of the map you'll see this:

Click "Export Current View".

4. Save the image as an GeoTIFF file. Has to be GeoTIFF as that embeds the georeferencing information within the image. Don't use compression as Ozi doesn't support it.

Select whatever size you like. I recommend 100%.

That's it. The map will be exported. There is no confirmation message, just go to Step 5.

5. Run Oziexplorer and select Import Single DRG Map.

You'll see this:

Click ok, and find the file you saved in Step 4:

If you get this error:

then either you haven't saved the file as a GeoTIFF, just as a plain TIFF, or you're missing OziGeoTiff.dll from your Oziexplorer directory, usually c:\oziexplorer. Get OziGeoTiff.dll from the Oziexplorer Australian Page, or directly from this link: http://216.218.220.254/xtras/ozigeotiff.zip. Unzip it into the same directory as Oziexplorer.exe.

5. Now you'll see this:

Click ok, and then create yourself a mapfile. The GeoTIFF format stores the georeferencing data within the image file, whereas Ozi stores it outside the image, in a separate file called a mapfile with a .map extension.

Keep the import parameters as the default. Check they are the same as these:

and you should see this:

6. You now have a suitable .map file for your subdivided map image.

If you want to use this new map on your PocketPC with OziCE, then you must:

7. Convert the image file to Ozf2, using Img2Ozf2.

8. Optionally edit the .map file so it points to the new ozf2 image rather than the original GeoTIFF. From Oziexplorer, take the File Menu, then Change Image File Name, Path and Drive.

9. Copy the .map file and the .ozf2 file to your PocketPC.

Notes

1. It would in theory be better to do this as an ECW export instead of GeoTIFF. However, the Oziexplorer website warns against this because of potentially incorrect georeferencing information created during the export. I've also found this problem. So stick to GeoTIFF.
2. For interest, here are some relative filesizes.

   

   Your results will vary depending on the Img2Ozf paramters you select and lots of other factors. The file above was created with the defaults.

3. You can use ER Viewer to do the GeoTIFF export if you prefer. ERViewer is a handy, free, utility from http://www.ermapper.com/. The user interface is perhaps better, certainly different. Thanks to Tony Boocock for reminding me about ERViewer.

Resources

• Geoscience Australia Raster 250k Website
• GPS OZ's Natmap Page
• Oziexplorer's Australian page, which covers the Natmaps
• Oziexplorer's Natmap Page: which to buy
• Oziexplorer's How to Subdivide the Mosaic page
• TrackRanger website.

Feedback? Contact me.

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