Lesson 2. Setting UCS, eliminating
distortions using calibration
Brief
description: During
this lesson you will learn how to set up a User Coordinate System (UCS),
and eliminate distortions of a raster image using calibration.
 See section Calibration of
the Quick Start guide for detailed information.
Setting a new origin
- Specify the coordinate of the bottom-left grid node on the raster
image.
- From the AutoCAD Tools menu choose New UCS, and then Origin.
- Use the cursor to specify the bottom-left grid node.
|
 |
Calibrating raster images
Calibration is used to eliminate arbitrary (both linear
and non-linear) distortions of raster images.
The original image must contain points with known coordinates. These points
are called real points. You
can use the nodes of a rectangular grid as real points (if you work with
mapping materials) and/or other points, whose coordinates are known or
can be calculated.
The points that correspond to the real ones on the distorted image, are
often located with some displacement. These points are called measured
points, as their coordinate value can be measured on the raster image.
The purpose of calibration
is to transform the raster image so that the measured points coincide
with the real points.
Define real
points
You can use
the rectangular grid nodes as real points on this image.
A grid is defined by three parameters:
start point (coordinates of bottom-left node), cell size (by X and Y axis),
and grid size (by X and Y axis).
Define the calibration grid:
- From the rImage menu choose Calibration
and then – Calibration Grid.
- In the AutoCAD command line, specify:
- Grid size by X and Y axis
- Origin (coordinates of bottom-left node)
- Cell size by X and Y axis.
|
 |
Specify measured points
From the rImage
menu choose Calibration and then – Repick Calibration Point.
WiseImage will highlight the first specified real point (point 1) with
grips.
- Move the grips to the corresponding grid node on the scanned
image (point 2) – this is how you specify the measured point.
- Press N
(next) to move to the next calibration pair. (P
(previous) – to move to the previous one) and press Enter. Specify measured points for
all real points from the list.
- Exit from Repick mode by pressing X (eXit).
|
 |
Calibration object; running calibration
- Select the calibration object by clicking on it.
- Open the AutoCAD Properties toolbar (by choosing Properties from the Tools
menu).
- From the combo-box of Calibration Method select Choose automatically or,
if needed, any other transformation method.
- If you choose Yes in the Auto Estimation field, you’ll get
a message about the error in the chosen calibration method (in
the Min
Error, Max Error, and Mean Error fields). You can also
sort out the points (choosing a required criteria from the Point
Sorting list), and view them in the selected order using
the arrows of the Number field, etc.
|
 |
- Run Calibration – from the rImage menu choose Calibration
and then – Run Calibration.
|
Saving the Document
- You can save the calibrated image by choosing Save or Save As from the rFile
menu.
- You can save the AutoCAD document with the calibrated image using
Save
or Save
As from the File menu.
|
