Like all maps on this site, the maps are raster maps - dots rather than vector data - which means you can work on them in exactly the same way as you work on your scanned images or digital photos.

Topographic Maps & Historical Survey Maps - Colour Depth

The maps vary from 12 to 62 megapixels in size. To permit a practical download file size, they are supplied as 8-bit/256 colour .png files. This means that you will be able to cut, crop, and paint the maps with you own data, but if you want to carry out global colour operations such as adjusting brightness or applying special effects you will need to increase the colour depth (usually to 24-bit/16 million colours). Most picture-editing software can do this, but if colour operations are important to you we suggest you confirm the capability of your software first.

If there is no explicit colour depth command, try a "save as" option. Saving as .jpg or as .png "true colour" will automatically increase the colour depth. If saving as .jpg, use the highest possible quality setting, and don't discard the download file.

The maps have been designed from the ground up for ultimate reduction to 8-bit colour depth, and there is no noticeable colour difference between the download files and our 24-bit "mother files".

Memory Requirements

The maps are large and you will need a generous amount of memory to handle the bigger ones.

For these 8-bit/256 colour maps, the memory you need is approximately pixel dimensions x 3. If you want to increase the colour depth to 24-bit/16 million colours so you can carry out colouring operations, then multiply by another 3. For example, if you just want to open and print the New Zealand Regional Basemap (ABMR14, the smallest Basemap), you will need a modest 3360 x 4211 x 3 = 42 MB, say 64 MB. Not many computers have less than this these days, but, at the other end of the scale, if you want to do all colour operations on our Senior World Map (the largest Basemap) you will need 10800 x 5724 x 3 x 3 = 556 MB, say 768 MB.

In summary, if you have a 128 MB or 256 MB machine, you should be able to do anything with all maps except the Senior World Basemap. With a 512 MB machine, you will be able to do 24-bit operations on the same maps. To do colouring operations on the "Senior World", you need 768 MB.

We emphasise that this is an approximate guide - a lot depends on how memory-hungry your software is, and you can reduce memory requirements by reducing the number of "undo" steps.

Preset Resolution and Printed Map Size

The download files are supplied with the resolution pre-set for printing on A4- or Letter-sized paper. This resolution is much greater than required for a sharp print, and you can print a much larger map by changing the resolution (dividing pixel dimensions by resolution gives printed size). Most picture software can do this.

Publication-quality resolution is generally taken to be 120 dots per cm/300 dots per inch. Our maps still look sharp at 80 dpc/200 dpi. At this resolution, even the Junior World map will provide a 67 x 36 cm print. Your printing size and resolution options are therefore likely to be limited by what size of printers you have access to.

If you do not have access to a large-format printer, don't despair, you can make a perfectly presentable large map by printing the map in panels and joining them carefully with sticky tape.

Cutting-Out and Resizing

The minimum printing resolution we recommend is 60 dpc/150 dpi. On this basis, if you want to fill an A4- or Letter-size page with your map cut-out, for which you need an image of about 25 x 18 cm, your cut-out should be at least 1500 x 1080 pixels in size. If you need to enlarge the part of the map you have cut out, the bicubic enlargement tool offered by most popular photo-editing software is fine for enlargements of up to about 50% in linear dimension. This means you cut-out can be as small as 100 x 720 pixels for an A4- or Letter-size print.

If your area of interest is even smaller than this, and you still want a big print, you would best use specialist enlargement software. We can report that the most cost-effective software of this type that we have so far come across to date is the "Stoik Smart Resizer", around $50 from www.stoik.com (use the "smart for artificial scenes" enlargement option).

How The Maps Are Made

The maps are based on USGS global terrain height data which is projected by software into 2D format using the Mercator method, and rendered to colour-code this data and produce a shaded relief appearance.

All these maps are "pixel sharp" - each map has been made separately, no map is an enlargement or reduction of any other, and no map has been enlarged or reduced from the native "database" size.

The Databases

We have several USGS terrain height databases, at different sampling intervals from 1 km to 10 km. The Decorative World Maps are based on a 10 km sampling interval; the World Basemaps on 8, 6, and 4 km intervals (Junior, Intermediate, and Senior maps respectively); the Continental Basemaps on a 2 km interval; and the Regional Basemaps on a 1 km interval.

These "km" figures are approximate - see the "Map Scale" section further down for an explanation.

"Pixel Size"

In the Basemap descriptions, to indicate the level of detail of the maps we use the term "pixel size". This is simply the ground area covered by each map pixel at the equator. For example, the pixel size of the 1 km Regional Basemaps is approximately 1 km x 1 km = 1 sq. km; at the other end of the scale, the pixel size of the Junior World Map is 8 km x 8 km = 64 sq. km. In other words, for any area on the globe there is 64 times more topographical detail in our Regional Basemaps as in the Junior World Map.

Map Accuracy

The maps are designed for illustrative rather than navigational use. We have no reason to disbelieve the accuracy of the terrain databases or the rendering software, but the fact that they are raster maps leads to approximations. Each pixel represents average land height for the land area defined by "pixel size", and where there is little difference in heights between adjacent pixels there may be incorrect colour assignment. This is particularly possible in low-lying coastal areas. However, any anomalies that might exist at pixel level are not obvious when the maps are printed at normal printing resolution (80 dpc/200 dpi and up).

Map Editing

In the interests of veracity, we minimise the changes we make to the topographic details of the maps. Where we think corrections are essential, we check with details shown in standard reference atlases. At present there are four major locations where we have overidden the software output. These are parts of the Netherlands below sea level (we have added polder areas), the Caspian and Aral Seas (we have used recent outlines from official sources), and a small area in the eastern Himalayas where there is an anomaly in the data (pending fresh data, we have cloned a patch from another area with similar stratigraphy). The latter revision has been applied only to maps ABMC05 (Continental - Asia), ABMR10 (Regional - Asia) and ABMR12 (Regional - East Asia): the revision is too small to show up on the World Basemaps and Decorative Maps.

World Map Layout

The world maps are centered on 10°E rather than the conventional Greenwich meridian to keep Siberia together and to give New Zealand more sea room.

You can easily produce a "Pacific-centered" World map by cutting out the "American" left-hand side of the map and pasting this on the right-hand side. We suggest you cut at the 30°W meridian - see the latitude and longitude details provided with each map to precisely locate this.

Map Scale

The scale of a Mercator map varies with distance from the Equator, and the best way of establishing the scale of any part of a map is to measure the pixel distance between features which are a known distance apart.

The equatorial scale of these maps can be established by dividing the pixel distance along the Equator by the real distance. The pixel distance of our 1 km database is 43200 pixels, and pro-rata for the other sampling intervals. The real distance is 40075 km. On this basis, the equatorial scales of the Regional, Continental, Senior World, Intermediate World, and Junior World Basemaps are (to the nearest pixel) 1078, 539, 270, 180, and 135 pixels per 1000 km, and the equatorial scale of the Decorative World Maps is 108 pixels per 1000 km.

Joining the Basemaps

If you wish to join any of the Continental or Regional Basemaps, you might need to slightly adjust the brightness and contrast of one or other of the maps. This will need to done separately for the land and sea areas - use your "magic wand" and the "invert selection" command to juggle between the two.

Ask Us

We do not have a FAQs (Frequently-Asked Questions) page on this site because we think we have anticipated most questions here. If you want to know more about resolutions, paper, ink, and other related topics, visit the FAQs page on our picture site - there is a link on the "Welcome" and other pages here. Alternatively, just ask us at maproom@avalon-download-maps.com.

We are always happy to hear from you and answer your questions.

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