Calibration  (georeferencing) of maps and orthophotos

What is calibration?

Large scale maps which can be used for accurately locating the features visible in an aerial photograph  are remade at intervals, and their geographic grids have been revised over time by the map makers. Data from satellite or aerial image servers are revised even more frequently.  Positional data determined by geodesy is more or less forever in terms of human life span, whereas field boundaries seen from the air change yearly in terms of the way they look and so-called fixed points visible in an aerial photograph can vanish at the flick of a lever on a bulldozer. Geodesy, meaning the accurate measurement of the shape of the Earth, used to be determined by astronomy, and now it's determined by accurate GPS measurement. Field boundaries are determined by agricultural requirements, land prices and the needs of farmers etc. and should never be considered as fixed.

The Earth is an ellipsoid which has to be "flattened" so that coordinate grid lines can be drawn in two dimensions instead of three.  

Any point is defined by its longitude and latitude on the Ellipsoid, but these depend on the constants used to define the shape of the Ellipsoid which has been re-measured many times in the past. Differences in the values of longitude and latitude with different ellipsoid constants (Datum) may give results which differ by as much as a kilometre.  They are now based on accurate satellite observations, but older values from astronomical observations in the first half of the 19th century are still in extensive use especially in large scale cadastral property boundary maps because of the costs involved in remaking so many of them.

All maps are based on a grid system. There are many grid systems which have been invented during the past several centuries in nearly all countries.  Several thousand are or were in existence all over the world.  AirPhotoSE uses the Universal Transverse Mercator grid with the NASA-determined Ellipsoid of 1984, UTM-WGS84 and corresponding longitudes and latitudes.  In AirPhotoSE, longitudes west of Greenwich and latitudes south of the Equator are negative decimal numbers.

In the UTM grid, there are 60 northern and southern zones covering the planet.  A grid reference for a point is given by a zone number, an "Easting" (X) and a "Northing" (Y) plus annotation to specify if north or south of the Equator. Points South and West are stored as negative numbers.  For backward compatibility with regional or country-specific grids, a simple X-Y coordinate system is also offered.  AirPhotoSE does not convert directly between different datums like AirPhoto can, but you can convert from a grid to UTM and from UTM to another grid in two steps.  

If you don't know the zone number for a place of interest and you have installed Google Earth (GE), you can get the number for any point quite simply. In the GE Tools, Options dialogue box, simply check:

Enable the Status Bar in the View menu, then, in the GE search field, enter the name of the place of interest and the image will zoom there.  In the middle of the status bar you will see the zone number followed by the easting and northing values of the centre of the search area:

However, some of the GeoPortal web sites which can be viewed in a web browser or from within AirPhotoSE offer several different grid systems and some offer a choice of corrected vertical aerial photographs (orthophotos) and maps from various periods and combinations of them.  Here is an example:

This shows a set of grid squares using the UTM-WGS84 standard superimposed on a map of roughly 10 square kilometres with modern field and property boundaries aligned to an earlier standard chosen by a national mapping service in the 19th century and still used for cadastral survey in some areas.


AirPhotoSE UTM-WGS84, Latitude/Longitude and X-Y (E-N) grids from the Options menu and over 3000 grids using the EPSG option.

Steps in Calibration:

From the main menu, choose Options, then choose the second tabbed page and select a desired grid from the choices shown in the upper right corner.

Choose a grid:

Then, open a map, satellite or vertical aerial image and after it loads, click on the Calibration menu option to show:

The Calibration Menu:

Choose Calibration from the main menu.

Large scale maps or satellite or vertical aerial images are often slightly distorted.  AirPhotoSE can correct for shift, rotation, scale and skew which may be introduced by scanning a paper map or using a satellite or uncorrected vertical aerial image.  For this purpose, at least four points with known coordinates in one of the three grid options offered must be entered interactively.   Entering more than four points improves the accuracy of the correction.  The points may be anywhere in the image, but for best results they should be as widely spaced as is possible.  

Calibration Point Entry:

Click on:

Enter/Delete Cal Point

Entering a point is made easier by the programme's use of a movable magnifying cursor with cross-hairs for accurate positioning over visible features.

A click on a point with known coordinates will cause a dialogue box to pop up into which you must enter them.  

There are four grid options, and each requires different information.  Choose the grid option which is appropriate for your choice of map, orthophoto or satellite image.  

If the image to be calibrated comes from a GeoPortal web site, there may be different grid options available. It is easiest to use UTM or a national grid. If you choose decimal latitude/longitude or degrees/minutes/seconds because nothing else is available, errors in numerical entry in AirPhotoSE may occur when 6 or 7 digit values are entered manually.  If the GeoPortal offers an option to set markers at highly visible points or at a grid if shown, and if these markers can be labelled with their numerical vales or exported to a text file as described in the next chapter, this helps to overcome problems.  Each GeoPortal has it's own rules and features, so they should be studied before using one in AirPhotoSE with calibration.

The UTM option requires specifying whether or not the area of interest is north or south of the Equator, the UTM zone number which you can select from the diagram shown above and the grid  values for "Easting" and "Northing".

The Longitude/Latitude option requires specifying whether the area is north or south of the Equator or east or west of the Greenwich meridian.  Negative values may be entered when needed or one can rely on the choices made at the left side of the dialogue box.

Longitude and Latitude may also be specified in Degrees, Minutes and decimal Seconds.  This is useful when entering calibration points from GeoPortal sites which show coordinates in this form only.  

X or East values and Y or North values for any area which only has a single grid without zones may be entered as shown.

After clicking OK a marker will be written in a layer over of the map. 

and moving the magnifier will permit you to see it. If you have made a mistake, simply move the magnifier over the point and click again.  The marker will be erased.

After you have entered four points, the current coordinates for the mouse cursor in the chosen grid are shown in the bottom right corner of the screen:

Off-screen Point Entry:

Click on:

Intersection for Cal Point

If you do not have a map which extends far enough, but on which you can see two straight line features such as roads which appear to converge outside the image, you may choose to place a calibration point at the intersection of two lines drawn over the converging features.  The cursor changes to a little page with an arrow.  Hold the left mouse key down and move it then release the mouse key to make the first line and repeat this for a second line.  The programme calculates the intersection point even when only short line segments are drawn over line features  and places it outside the border of the visible image if necessary.

As with single point entry, the dialogue box appropriate to the choice of grid will pop up and after clicking on OK, a square double marker will be written near the edge of the image to indicate that an off-screen calibration point has been placed in that direction.  As before, you can delete this point by clicking on it with the Entry magnifier if you have made a mistake.

Disabling a point:

You may wish to disable a calibration point to see the effect on the accuracy of calibration elsewhere on the map. 

Click on:

Disable/Enable Cal Point

The Entry magnifier rim colour changes to blue and clicking on a visible calibration point will disable it.  This remains in effect even after you close the image and turn off your machine.  Single calibration points can be re-enabled by clicking on them with the same magnifying cursor.

Enable all disabled points:

Click on:

Enable All Cal Points

You can re-enable all disabled points with a single click on the Enable All menu item.

Load and save to the calibration file:

Click on:

Load Calibration

or click on:

Save Calibration

Data for calibration is stored in a disk file with the name of the image and the extension ".cal" when the image is closed.  This file contains the screen coordinates of all calibration points, their longitudes and latitudes, an identification number and the kind of coordinates which has been selected.  You can load a previously created calibration file or save it separately to a file with a different name.

Edit calibration values:


Edit Calibration

You can display and edit all the calibration values in a table.  The editing table is loaded from the image by default, but you can also load from a disk file in one of several formats and save them as well.  

Delete all calibration:

Click on:

Delete Calibration

You can delete all the data in the currently opened calibration file.  You will be asked if you really want to do this.

Hide or show calibration points:

Click on:

Hide/Show Calibration

You can hide or show all entered calibration points with a single click on this menu option.

Add a Grid:

Click on:

Add Grid

If the image has four or more calibration points, it is said to be calibrated, and you can display a grid overlay showing lines or fiducial cross points along with the numerical values of the grid.  The default colour is red and the line thickness two pixels, but you can change this along with the text colour.

You can hide or show the grid with the Undo button on the main toolbar.    You can change the grid interval if desired or draw a rectangle around the grid, indented by up to 10% of the side length of the image.  The numerical labels for the horizontal lines of the grid may be drawn vertically at the sides if the vertical check box is checked.

As an example of a result, the image below shows a UTM grid superimposed on a UK Ordnance Survey map: