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Didger 3: Changing the
Projection of an Image with an ESRI World File
Introduction
Georeferenced TIF files commonly save the spatial
reference information in an ESRI World File (*.tfw). You may want to import the
georeferenced TIF file using the coordinates in the ESRI World file into Didger
for a number of reasons such as warping the image, mosaicking the image with
other GeoTIFF files, digitizing the image, automatically creating a Surfer base
map, or overlaying other data or vector files on top of the image.
However, sometimes it is necessary to change the
projection of an image georeferenced with a TFW file in order for the
coordinates to match up with other vector, data, or georeferenced image files.
For example, the image may be calibrated in UTM units and you want to overlay
the image with other data that is in Latitude/Longitude units and you want the
resulting map to show Latitude/Longitude coordinates.
A bitmap georeferenced in UTM
units will not overlay correctly with data
that is georeferenced in Latitude/Longitude units.
Changing the Image's Projection
You cannot directly convert the projection of the
image from UTM to Latitude/Longitude because the TFW file does not contain any
projection information. TFW files only contain coordinate information. Here are
the steps you can use to assign a projection to the georeferenced image file,
convert the projection, and overlay it with other data files:
- Start Didger.
- Go to File | Import Bitmap | Into Raster
Project.
- Select the TIF file and click Open.
- In the Bitmap Import Options dialog
box, select the spatial reference source to be ESRI World File and click OK.
- Click OK in the second Bitmap Import
Options dialog box.
- Once the image is imported, go to Image |
Calibrate Image. In the Initial Calibration Settings dialog, set
the Coordinate Space Type to Projected Coordinates, then choose the
appropriate projection parameters for the image.
Assign the appropriate
projection for the bitmap in Step One under Image | Calibrate Image.
- Click Next to go to Step Two of the
calibration process, the Raster Calibration dialog. The coordinates
shown here were imported from the TFW file. Do not change anything and just
click Next, Next, and Finish.
- The image is now calibrated and projected.
Before the projection of the image is converted, it is recommended that it
is warped first. Go to Image | Warp Image and click OK.
- Go to Image | Convert Image Projection
to convert the image to any other supported projection. After setting the
desired projection parameters, click OK to change the image's
projection.
Use the Image | Convert
Image Projection command to convert
the projection of the bitmap from UTM to Latitude/Longitude.
- Go to File | Export and choose to
export the image as a Tagged Image (TIFF) (*.tif) file. Give the file a name
and click Save.
- In the Tagged Image Export options
dialog box, choose to save the spatial reference information in Embedded
GeoTIFF format.
- Go to File | New | Blank Project.
- Go to File | Import Bitmap | Into Vector
Project.
- Select the GeoTIFF you just exported in Step
10 above and click Open.
- Select Embedded GeoTIFF Parameters as the
Spatial Reference source and click OK.
- Now you can go to File | Import Vector,
File | Import Data File, or File | Import Bitmap to import
other Latitude/Longitude data on top of this image. Alternatively, you can
go to File | Create Surfer Base Map and Didger will open Surfer and
load the georeferenced image automatically into Surfer as a base map.
Once the bitmap is
georeferenced in Latitude/Longitude coordinates,
it will overlay correctly with other Latitude/Longitude data.
If you have any questions about Didger or this
article, please contact us at didgersupport@goldensoftware.com.
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