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Import & Calibration of Bitmaps

Bitmaps serve as a background graphic when entering sources, obstacles and receiver points. In those cases, the object coordinates are not available as digital data but may be defined by drawing the objects onto the background. This feature enables to make use of scanned plans or maps (e.g. from land register), but also of photographs serving as background.

Note

The bitmap file itself is not saved within the CadnaA-file. The CadnaA-file, however, contains a reference to the file via its file path.

It is mandatory for the to-scale insertion of a bitmap file that one of the following data is at least available:

  1. coordinate pairs (x,y) for the bottom left and the top right corner,

  2. coordinate pair (x,y) for bottom left corner, the resolution in dpi ("dots per inch") and the original scale (1: X),

  3. at least two matching points (located within the bitmap with well-known coordinates) assuming translation deformation, or

  4. at least three matching points with a shear deformation of the bitmap.

Example 1

Example

Path: Bitmaps/bmp1.zip

The bitmap-file bmp1.jpg is an extract from a land register. It is orientated vs. North and shows a scaling bar.

The scaling bar has a length of 25 m. Thus, the relative, orthogonal coordinates of two points are known:

(x1,y1) = (0,0) and (x2,y2) = (25,0).

  • Select the bitmap-symbol from the toolbox and draw an area.

As no bitmap-file has been referenced so far the frame just contains the default bitmap-symbol.

  • Open the bitmap’s dialog (in input mode) by clicking on the border with the right mouse button.
  • Select the bitmap-file to be imported bmp1.jpg using the file selector symbol.

After selection of the file the entire path is displayed in box „File“.

Note

A bitmap-file is, however, found in any case - independent of the file path addressed - if it resident in the same directory as the project file (file extension *.cna).

The present coordinates for the two corners apply to the drawn area of the bitmap-symbol.

  • Click on button „Calibrate Bitmap“.
  • On dialog Calibrate Bitmap, the option „Reference Point 2“ is selected by default.
  • Now, enter the x-coordinate of the second reference point as shown in the next figure.

The remaining coordinates are all zero and need not to be changed.

Coordinates for two reference points

  • After having defined the coordinates click on OK.

On a further dialog, CadnaA displays the bitmap and prompts you to specify the position of the first reference point. When moving with the mouse arrow across the bitmap you will realize a fiducial mark attached to the mouse arrow.

Symbol for the fiducial mark

Note

The hotspot is the mouse’ arrow head and not the center point of the fiducial mark.

  • Click onto the zero meter point on the scaling bar in order to enter the first reference point.
  • If necessary make use of the magnifier feature to enlarge a section of the bitmap.

Now, CadnaA prompts you to specify the second reference point.

  • Click at the 25 m point on the scaling bar to introduce the second reference point.

Defining the second reference point at (x2,y2) = (25,0)

After input of the last reference point a further dialog displays the RMS-error and the standard deviation of the calibrating procedure. For bitmaps with two reference points the error with an orthogonal transformation is zero.

  • Click OK and enter a new file name for the calibrated file (e.g. bmp1_cal.jpg).
  • Now, click the OK-button in the Bitmap dialog.

The bitmap is imported, but may be out off the screen area as it’s location is outside the present screen area of the CadnaA-main window.

  • In case this applies, select the command Limits on the Options menu.
  • On the dialog Limits click the button "Calc" and close the dialog by OK.

  • Click the symbol on the toolbox to display the entire limits.

  • By use of an auxiliary polygon you may check whether the scaling bar actually has actually a length of 25 m.

Example 2

Example

Path: Bitmaps/bmp2.zip

In this example the bitmap-file bmp2.bmp having a shear deformation is imported to scale using three match points.

Bitmap with three match point-coordinates

When importing a file with an unknown distortion use at least three match points.

  • If necessary select the command New from the File menu.
  • Draw an area after selection of the bitmap symbol

from the toolbox.

  • Open the dialog (in the edit mode) by a right mouse click on the border of the bitmap symbol.
  • Select via the file selector symbol the bitmap file bmp2.bmpto be imported.

Upon selection of the file the entire file path and the file name is displayed in the box "File". The coordinates displayed right now apply to the two polygon points of the arbitrary area of the bitmap symbol.

  • Click the button „Calibrate Bitmap“.
  • In the dialog „Calibrate Bitmap“ activate the radio button at "Reference Point 3" to enable the input of three reference points.
  • Enter the coordinates of the three reference points as shown in a following figure.

Coordinates for three reference points

Note

It is recommended to apply a certain sequence when defining the reference points (e.g. clockwise) in order to be able to remember the sequence of points more easily - if a sequence is not visible from the bitmap file already.

  • Click after input of the reference point coordinates on OK.

CadnaA displays the bitmap and prompts to enter the position of the first reference point.

  • Click on the location which was entered as the first reference point.
  • If necessary use the magnifying glass to zoom to a part of the bitmap.

CadnaA prompts you to enter the positions of the two further reference points.

  • Click at the location which was entered as the second reference point.
  • Further, click at the location which was entered as the third reference point.

Defining the third reference point

After input of the last reference point the RMS-error and the standard deviation of the calibrating procedure are displayed. For bitmaps non-translatoric distortion - as in this case - the error applying an translatoric transformation is, for sure, larger as with an affine transformation.

RMS-error and standard deviation of the calibrating procedure (applying orthogonal or affine transformation)

  • Click OK with the a affine transformation selected and enter a new file name for the calibrated file (e.g. bmp2_cal.bmp).
  • Click OK on the dialog Bitmap.
  • If the bitmap is out off the screen area select the command Limits on the Options menu.
  • Click the button "Calc" and close the dialog by OK.

  • Select the symbol from the toolbox to display the entire limits.

  • Using an auxiliary polygon you may check whether the distances between the locations having served as match points correspond to the differences of the respective coordinates in x and y direction.

Bitmaps are also listed in the tables (see on the Tables menu the sub-entry Miscellaneous|Bitmap. The calibrated bitmap enables to draw objects - like buildings and roads - on top of the bitmap serving as background.

Note

The imported bitmap file is projected to the ground in 3D-Special view.