ReferenceTool

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Referencing maps
- Reference tool
  - Basic information
  - More details
- Example: Referencing of Russian military maps

Referencing maps

Raster maps are typically obtained by scanning maps or collecting aerial photographs and satellite images. They are saved as image files in common formats such as TIF, JPG, PNG (QMT can handle all image formats for which GDAL provides a driver, a list can be obtained by calling gdalinfo --formats, prefer the TIFF format).

(Geo-)referencing a raster map means

to add reference points to the map that link known raster positions to known positions in map coordinates,
to save the georeferencing information when the referencing is satisfactory (output format, for instance, GeoTIFF).

QMT's reference tool provides an easy way to reference raster maps.

Reference tool

Basic information

To work with the QMT reference tool proceed as follows:

Select the menu entry Window - Shell to open the QMT shell window (docked window, can be moved on the desktop).
Find more than 3 reference points in the raster map with known WGS84 lat/lon coordinates. Remark: An easy way to get reference points with coordinates is to use a map (e.g. OpenStreetMap) in QMS.
Click the Reference tool button in the QMT Tools window (docked window, can be moved on the desktop).
Click the Open (Add map files to list) icon () and select a raster map. The raster map is loaded into the QMT map window.
Click the Add reference point icon (). The map cursor now is a cross-hair one.
Move the mouse cursor on each of the known reference points as precisely as possible (use mouse wheel to zoom map!) and left-click to fix the reference point.
Insert the WGS84 coordinates of the reference point into the pop-up window. The input format is shown as soon as input is started. A small black square is shown at the reference point on the map. The coordinates and the pixel location of the reference point are inserted into the list of reference points in the tool window.

Remark: An easy way to get reference points into QMT is to use QMS. Find some reference point which is on the raster map and the QMS map as well. Right-click the QMS map at the reference point location to get a context menu and then click Copy position. This copies the current position to the clipboard. Now, go to the QMT window and add the reference point. When being asked for the coordinates simply press CTRL-V to insert the coordinates found in QMS.
Click the Start projection wizard icon () and select the coordinate system for the output of referenced data. An example is shown in the next image.
Leave defaults for other settings.
Click the Start button to get a referenced image file and a VRT file in the directory of the original raster map. The shell window displays the GDAL commands run.

More details

In addition to the basic steps described in the previous section some more actions with maps and reference points can be carried out in the reference tool:

Icon	Tooltip	Comment
	Remove selected (map) file from the list	Reference points are deleted, too.
	Clear complete list of map files	Reference points are deleted, too.
	Reload the currently selected map	Reference points aren't changed.
	Move the map and zoom	Zoom with mouse wheel.
	Add reference point	Move mouse to wanted point, left-click and insert coordinates into the coordinate definition window.
	Move reference point	Reposition misplaced reference point. A selected reference point either in the list or in the map changes the square color to red. Left-click the reference point on the map, move it to new location, and left-click again to fix it.
	Remove single reference point	A selected reference point either in the list or in the map changes the square color to red. Left-click in the map to remove it.
	Move reference point with automode. This will pick up (move the mouse to) the next point after you moved a reference point.	Press `b` or `n` on the keyboard to jump to the previous or next reference point in the list.
	Remove all reference points	Requires additional confirmation of deletion.
	Switch to the grid tool	Reference tool add-on for fast creation of reference points if map has known grid.
	Load reference points from GCP file	Load previously defined and saved reference points from file.
	Save reference points into GCP file	Save reference points for future use.
	Sort list of reference points

If the field labeled Embed result into *.vrt file is not checked, then the above-mentioned VRT file (needed for displaying the map in QMS) is not created. Remark: Leave the checkbox selected, if the referenced map should be used in QMS.
It is possible to open several map files and to reference each of these maps. If the checkbox For all files at the bottom of the tool is checked, then the referencing is carried out for each of the loaded map files.

Remarks:

The Create overviews ... option is available in 4 QMT tools (cut tool, overview tool, reference tool, palette tool). This is necessary because there is no fixed order of using these tools. When one of these tools calls GDAL for some operation, then GDAL doesn't read the overview information contained in the raster map file. Thus, overview information should be recreated after such a GDAL operation and this is supported in each of the tools.
Experienced users may change some GDAL default parameters to equilibrate performance, image file size, and image accuracy using a special group box for GDAL parameters. For details compare the page "Optional GDAL settings".

Example: Referencing of Russian military maps

http://loadmap.net is a server for a wide variety of raster maps covering large parts of the world. Among them are various kinds of Russian army and topographic maps.

These maps have a large boundary for map metadata information.

Typically, Russian military maps are georeferenced with reference information provided in the OziExplorer MAP format. As a rule, the Krassowski ellipsoid (epsg 4284, Pulkovo 1942) is used in the MAP file for lat/lon coordinates. In this example, it is assumed that the differences between these coordinates and the WGS84 lat/lon coordinates used in the reference tool can be neglected.

Using a text editor and the information from the MAP file it is easy to cut and reference such maps with QMT. Follow these steps for downloading and referencing a (cut) map:

Select a map on http://loadmap.net.
When downloading the selected map, 2 files are offered: a GIF and a MAP file. Download both files and put them into some directory.
As an example, the files 001m--m36--(1989).gif, 001m--m36--(1989).map will be used in this discussion.
Open the MAP file in a text editor and find the lines labeled POINT near the top of the file.
Assume the MAP file has non-zero values in the latitude and longitude columns of the line labeled Point01 (the other case is described at the end of this subsection).

In the example these lines are

  Point01,xy,  219,   86,in, deg,  52,  0.0000,N,  30,  0.0000,E, grid,   ,           ,           ,N
  Point02,xy, 1842,  121,in, deg,  52,  0.0000,N,  33,  0.0000,E, grid,   ,           ,           ,N
  Point03,xy, 3466,   90,in, deg,  52,  0.0000,N,  36,  0.0000,E, grid,   ,           ,           ,N
  Point04,xy,  147, 1839,in, deg,  50,  0.0000,N,  30,  0.0000,E, grid,   ,           ,           ,N
  Point05,xy, 1839, 1873,in, deg,  50,  0.0000,N,  33,  0.0000,E, grid,   ,           ,           ,N
  Point06,xy, 3533, 1842,in, deg,  50,  0.0000,N,  36,  0.0000,E, grid,   ,           ,           ,N
  Point07,xy,   74, 3591,in, deg,  48,  0.0000,N,  30,  0.0000,E, grid,   ,           ,           ,N
  Point08,xy, 1835, 3629,in, deg,  48,  0.0000,N,  33,  0.0000,E, grid,   ,           ,           ,N
  Point09,xy, 3598, 3594,in, deg,  48,  0.0000,N,  36,  0.0000,E, grid,   ,           ,           ,N

(use only lines where values are filled in!)

Convert this block with the editor to

  #V1.0 
  #gcpproj: +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs 
  -gcp 219 86 52.00000000 30.00000000
  -gcp 1842 121 52.00000000 33.00000000
  -gcp 3466 90 52.00000000 36.00000000
  -gcp 147 1839 50.00000000 30.00000000
  -gcp 1839 1873 50.00000000 33.00000000
  -gcp 3533 1842 50.00000000 36.00000000
  -gcp 74 3591 48.00000000 30.00000000
  -gcp 1835 3629 48.00000000 33.00000000
  -gcp 3598 3594 48.00000000 36.00000000

Remark: In the above block the fractional part of the geographical coordinates is obtained by dividing the seconds part of the geographical coordinate 60!

Save these lines to a new file with filename 001m--m36--(1989).gcp.
Start QMT and open the reference map tool in QMT.
Load 001m--m36--(1989).gif into the reference tool (hint: a map file previously cut with the cut tool can be used here, too).
Select the Load reference points from GCP file icon in the toolbar and load the file 001m--m36--(1989).gcp.
The map window shows now 9 reference points (4 points in the corners, 4 points in the middle of the 4 edges of the map, 1 in the center of the map).

If necessary, improve reference points manually as described on this page.
Define final map projection.
Click Start to create a new referenced map file.

For some MAP files, the described procedure may fail because there are no latitude/longitude coordinates in the Point lines. In this case, the described procedure for creating the GCP output file should be changed slightly as follows:

Find lines labeled MMPXY and MMPLL near the end of the MAP file.

An example of such lines is

  MMPXY,1,5,180
  MMPXY,2,4068,162
  MMPXY,3,4091,3915
  MMPXY,4,16,3933
  MMPLL,1,39.25,44.0
  MMPLL,2,39.5,44.0
  MMPLL,3,39.5,43.8333333333
  MMPLL,4,39.25,43.8333333333

The MMPXY,i lines give the values for the first two fields (pixel coordinates) in the new output line.
The corresponding MMPLL,i lines gives the longitude and latitude coordinates for the new output line.
Build for each such pair a new line of the form
```
  -gcp 5 180 44.0 39.25
```
(for the example i = 1 was used, pay attention that the output order is latitude - longitude!).
Proceed as described in the first part of this subsection.

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