Conversion and georeferencing of maps for ttMaps

Disclaimer

Before using a map with ttMaps, make sure you have the right! Some publishers of paper maps prohibit you to scan them, and vendors of maps on CDROM impose very restrictive licenses, such as the ban on the use of maps with other software than that provided by the vendor.

Maps format

The format that was chosen for ttMaps is the ECW format, developed by the company Er Mapper, recently acquired by ERDAS.. Why use such a choice? This format has many advantages for mapping applications:

It is a format chosen by the IGN to deliver maps and aerial photographs. It is therefore possible to directly use the files from IGN. See, for example, screenshots some of which were produced with samples of IGN maps.

Calibrating maps

Before you start converting the maps to ECW format, you must make sure they are calibrated (georeferenced), therefore you need to know:

In general, the datum and projection are shown in the legend text of paper maps. If you do not know the geographical extents, you must first calibrate the map. I will not explain in detail how to do this, because there are many programs available, such as:

Tools for conversion to ECW format

The main tools are:

Conversion of maps with GDAL

Here are three examples of using GDAL to convert the maps to the ECW format.

Georeferenced maps

Some file formats, such as GeoTIFF, include georeferencing data. For example, download the Sample GeoTiff file. After unpacking the archive, you can use the gdalinfo command to display information about the file: 

gdalinfo sample_geotiff.tif

Coordinate System is:
PROJCS["WGS 84 / UTM zone 10N",
    GEOGCS["WGS 84",
        DATUM["WGS_1984",
            SPHEROID["WGS 84",6378137,298.2572235630016,
                AUTHORITY["EPSG","7030"]],
            AUTHORITY["EPSG","6326"]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433],
        AUTHORITY["EPSG","4326"]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",-123],
    PARAMETER["scale_factor",0.9996],
    PARAMETER["false_easting",500000],
    PARAMETER["false_northing",0],
    UNIT["metre",1,
        AUTHORITY["EPSG","9001"]],
    AUTHORITY["EPSG","32610"]]
Origin = (563460.782789624412544,4180651.692221448291093)
Pixel Size = (14.250000000000000,-14.250000000000000)

This information allows us to know the reference system (here WGS84) and the projection used (here, UTM Zone 10 North). Unfortunately, Er Mapper uses its own names to designate the reference systems and projections. Before making the conversion, you must find these names.

Conversion of maps shipped with a world file

Some files, such as TIFF, JPG or PNG may be accompanied by a world file containing the geographical extents of the map. These files have respective extensions .TFW, .JGW and .PGW .

One example is the SCAN1000 map of IGN, which is freely available on the IGN site.

Download for example SCAN 1000 Corsica (Version Lambert II extended) and unzip the archive. The useful files are COR_0000.TIF (TIFF image) and COR_0000.TFW, which contains the coordinates and size of pixels. The gdalinfo command shows that this file uses a color palette: 

Band 1 Block=1200x25 Type=Byte, ColorInterp=Palette
  Color Table (RGB with 256 entries)

The gdal_translate utility is not capable of converting a file based on a color palette to a file in 16 million of colors. Fortunately, a conversion utility (pct2rgb.py) comes with the GDAL package. You must therefore begin doing this conversion: 

pct2rgb.py COR_0000.TIF CORSE.TIF

The website of the IGN tells us that this map uses the NTF reference system (New Triangulation French), with the Lambert projection II extended. The conversion command will be as follows: 

gdal_translate -of ECW -co "TARGET=88" -co "DATUM=NTF"  -co "PROJ=LM2FRANC" CORSE.TIF CORSE.ecw

However, we note that the map is surrounded by an unnecessary white border. It is possible to convert only a part of the map, using the -srcwin option of the gdal_translate command: 

gdal_translate -of ECW -co "TARGET=88" -co "DATUM=NTF"  -co "PROJ=LM2FRANC" -srcwin 63 84 980 1930 CORSE.TIF CORSE.ecw

For the Linux users (or Windows with CygWin), here's a script that converts the SCAN100 map.

Conversion of a map without georeferencing data

Download for example the image BlueMarble. The coordinates of the upper left corner are -180 °, 90 ° and those of the lower right corner are 180°,-90°. The datum is WGS84, and the projection is GEODETIC.

The command you can use for the conversion is: 

gdal_translate -of ECW -co "TARGET=94" -co "DATUM=WGS84" -co "PROJ=GEODETIC" -a_ullr -180 90 180 -90 land_shallow_topo_2048.jpg BlueMarble.ecw

Georeferencing of an ECW file

If you have a file in ECW format not containing the whole georeferencing information (datum, projection and extents), it is possible to add this information without having to decompress / recompress the file with gdal_translate. To do this, you may use the tool ECW Header Editor, that you can download at ERDAS.

To georeference the image, you must proceed as follows:

It is also possible to use gdal_edit , that works on Windows, MacOS and Linux.

Georeferencing a file already calibrated for OziExplorer

There are lots of maps for OziExplorer. Thus, we are going to show step by step, on three practical examples, how to convert your maps to ECW format.

How to check if the calibration file is correct

Often it occurs that calibration files for Oziexplorer do not contain the correct information on the projection and the datum. This is because some people use OziExplorer without sufficient knowledge, sometimes without even taking the time to try to understand the datums and projections.

There is a simple test to be carried out on a calibration file. The principle is to verify that the edges of the map are parallel to the axes of the projection.

To start, display the .map file contents (it contains plain text). In general, the file contains the coordinates of four control points, most often the four corners of the map. To ensure that the map is aligned with the axes of the projection, you just have to check that:

Example 1

Point Easting Northing
Coordinates of the upper left corner 0 2700000
Coordinates of the upper right corner 1100000 2700000
Coordinates of the lower right corner 1100000 1600000
Coordinates of the lower left corner 0 1600000

In this first example, we see that the map is well positioned. Indeed:

Example 2

Point Easting Northing
Coordinates of the upper left corner 4.269040 45.882684
Coordinates of the upper right corner 4.912734 45.870023
Coordinates of the lower right corner 4.891744 45.422152
Coordinates of the lower left corner 4.253713 45.434813

In this second example, we see that the map is misaligned. Indeed:

Note: This simple method is able to prove that the projection indicated in the .map file is incorrect. It does not check that the projection, datum and coordinates are correct, this should be done by displaying known points in ttMaps.

Tutorial n° 1 : Conversion of a french map coming with a correct calibration file

Here is an example, with a .map file found on a forum:

.map file lines Description
Map Datum,NTF France Datum
Map Projection,(II) France Zone II,PolyCal,No,AutoCalOnly,No,BSBUseWPX,No Projection
Image Width,44000 Image width, in pixels
Image Height,44000 Image height, en pixels
Point01,xy, 0, 0,in, deg, , ,N, , ,E, grid, , 0, 2700000,N Coordinates of the upper left corner
Point02,xy,44000, 0,in, deg, , ,N, , ,E, grid, , 1100000, 2700000,N Coordinates of the upper right corner
Point03,xy,44000,44000,in, deg, , ,N, , ,E, grid, , 1100000, 1600000,N Coordinates of the lower right corner
Point04,xy, 0,44000,in, deg, , ,N, , ,E, grid, , 0, 1600000,N Coordinates of the lower left corner

This table contains all the information necessary to create a georeferenced ECW file.

The easiest way to make the conversion is to use GDAL: 

gdal_translate -of ECW -co "TARGET=94" -co "DATUM=NTF" -co "PROJ=LM2FRANC" -co "LARGE_OK=YES" -a_ullr 0 2700000 1100000 1600000 map.png map.ecw

Note : The LARGE_OK=YES option here is essential, because the size of the map (44000 x 44000 x 3 = 5808000000 bytes) exceeds 500 Mb.

Tutorial n° 2 : Conversion of a french map coming with an incorrect calibration file

Here is a second example :

.map file lines Description
WGS 84,WGS 84, 0.0000, 0.0000,WGS 84 Datum
Map Projection,Latitude/Longitude,PolyCal,No,AutoCalOnly,No,BSBUseWPX,Yes Projection
Image Width,5000 Image width, in pixels
Image Height,5000 Image height, en pixels
Point01,xy, 0, 0,in, deg, 45, 52.96104,N, 4, 16.1424,E, grid, , , ,N Coordinates of the upper left corner (in degrees + decimale minutes)
Point02,xy, 5000, 0,in, deg, 45, 52.20138,N, 4, 54.76404,E, grid, , , ,N Coordinates of the upper right corner (in degrees + decimale minutes)
Point03,xy, 5000, 5000,in, deg, 45, 25.32912,N, 4, 53.50464,E, grid, , , ,N Coordinates of the lower right corner (in degrees + decimale minutes)
Point04,xy, 0, 5000,in, deg, 45, 26.08878,N, 4, 15.22278,E, grid, , , ,N Coordinates of the lower left corner (in degrees + decimale minutes)

We see immediately that the map is not aligned with the WGS84 projection. The main challenge will be to find the correct projection and datum.

As it is a French map, it's not very difficult: Most french maps use the NTF datum and the extented Lambert II projection (at the exception of some maps of Corsica that use the Lambert IV projection).

We'll check that by converting the coordinates of the four corners of the map into NTF/extented Lambert II. There are many softwares to convert coordinates, use whatever you prefer.

Points Longitude (WGS84) Latitude (WGS84) X (Lambert IIe) Y (Lambert IIe)
Upper left corner 4.269040 45.882684 750000.06 2099889.00
Upper right corner 4.912734 45.870023 799993.98 2099915.04
Lower right corner 4.891744 45.422152 799982.76 2050105.46
Lower left corner 4.253713 45.434813 750024.37 2050092.08

This map has probably been calibrated manually, and there are errors of a few tens of meters. After rounding results, we get:

Points X (Lambert IIe) Y (Lambert IIe)
Upper left corner 750000 2100000
Upper right corner 800000 2100000
Lower right corner 800000 2050000
Lower left corner 750000 2050000

So we see that the map is well aligned with respect to the axes of the projection NTF/extended Lambert II, and we can now convert it: 

gdal_translate -of ECW -co "TARGET=94" -co "DATUM=NTF" -co "PROJ=LM2FRANC" -a_ullr 750000 2100000 800000 2050000 map.png map.ecw

Note : The LARGE_OK=YES option is not useful, because the size of the map (5000 x 5000 x 3 = 75000000 bytes) does not exceed 500 MB

Tutorial n° 3 : Conversion of an english map coming with a correct calibration file

To understand this example, it is necessary to know the reference system used in the United Kingdom. Read for example http://en.wikipedia.org/wiki/British_national_grid_reference_system.

.map file lines Description
Ord Srvy Grt Britn,WGS 84, 0.0000, 0.0000,WGS 84 Datum
Map Projection,(BNG) British National Grid,PolyCal,No,AutoCalOnly,No,BSBUseWPX,No Projection
IWH,Map Image Width/Height,32704,32576 Taille de l'image, en pixels
Point01,xy, 200, 128,in, deg, , ,N, , ,W, grid, HV, 01000, 00000,N
Point02,xy,32400, 128,in, deg, , ,N, , ,W, grid, HW, 62000, 00000,N
Point03,xy,32400,32328,in, deg, , ,N, , ,W, grid, NG, 62000, 39000,N
Point04,xy, 200,32328,in, deg, , ,N, , ,W, grid, NF, 01000, 39000,N

This conversion is more complex than previous ones:

We will begin by converting the coordinates from letters + numbers to fully numerical coordinates. For that, it is enough to know that the squares measure 100km aside and that the reference is the southwest corner of the SV square (see map on Wikipedia). We deduce the following table:

Square X Offset Y Offset
HV 0 1000 km
HW 100 km 1000 km
NG 100 km 800 km
NF 0 km 800 km

that allows us to calculate the coordinates of the four control points:

Control Point of reference Easting Northing
Point01 1000 1000000
Point02 162000 1000000
Point03 162000 839000
Point04 1000 839000

We note in passing that the map is well aligned with respect to the axes of the projection.

Now calculate the coordinates of the upper left corner and lower right corner of the map.

We just have to convert the map: 

gdal_translate -of ECW -co "TARGET=90" -co "DATUM=OSGB36" -co "PROJ=TMOSGB" -co "LARGE_OK=YES" -a_ullr 0 1000640 163520 837760 map.png map.ecw

Last modification : 02/01/2014