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| Subject: | RE: [gislist] SUM: Projections |
| Date: |
10/21/2003 07:40:01 AM |
| From: |
Richard Nicoll |
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Additional:
The world file for a raster has a standard naming convention, using the
first and last letters plus a w. Thus:
Tiff = Tfw
Jpg = Jgw
Bmp = Bpw and so on...
When working with vectors (such as dxf) it is simply Wld.
The other option is to convert your image into a 'geo' file, such as
geotiff, geojpg, etc.
In this file format the raster cell co-ordinates themselves are the
projection co-ordinates, so there is no need to use world files. To
create a geo raster you must use a compatible package, I prefer ER
Mapper or ERDAS software. ER Mapper also allows you to use the Ecw
(enhanced compression wavelet) for imagery (as it is a lossy
compression) which can be useful when working with large images. Like
Mr-SID, it is not suitable for data such as LiDAR or DEM rasters as the
original cell values are altered/lost during compression.
With other types of imagery files it is also common to use a header file
(hdr) instead of a world file. This is a very similar method, but the
hdr can contain more useful information such as date and time of
acquisition, camera type, etc
-----Original Message-----
From: Taruna Tayal [mailto:tarunatayal@mabtrans.com]
Sent: 20 October 2003 19:54
To: gislist@lists.geocomm.com
Subject: [gislist] SUM: Projections
Thanks to all who responded. Following are the responses I received.
the TIFF file should have an associated world file along with it. that
is a text file containing the coordinates of the corners of the image. I
think the file extension is TWF? check to see if that file is in the
same folder as your TIFF image. if not, you can create one easily and
that should geo-reference your image.
If you don't have a *.tfw file to go with your*.tif then it isn't in a
projection. You will want to use the geoferencing toolbar in ArcMap to
project it/warp the image. As for the CAD file, for it to work in
ArcView 3.x, you need to turn the CAD reader extension on for it to be
visible in AV3.x. Although most times I find that CAD data isn't in a
projection, CAD people typically work in model space (drawing starts at
0,0), no projection associated with it.
Do you have a reference file for your tiff, ie a 'tfw' file.If you don't
have a reference file then you will first need to georeference using a
street file or similar in a known projection - either UTM or the Arizona
State Plane projection. Also NAD_83 refers to the Datum of a projection
system not a projection. 2) You will need to know the units that you are
working in - either meters or feet. This will make a difference. 3) If
your CAD file is an AutoCAD file try saving it as a Version 14 'dxf'. It
should then open in AV 3.2.
NAD 83 is not a projection, it is a datum. Is your projection in UTM or
State Plane, or what? You also need to know what measurement units were
used (usually feet or meters). You need to try to find out what
projection (if any) the CAD and tiff are in. If they line up with one
another, then they are in the same projection. What format is the CAD
file? The tiff should have a world file which gives the position of a
corner and the size of the cells. ArcGIS has some tools for aligning
images to vector features, but it is often easier to change the
projection of the shapefile - but for that, you need to know what the
projection is you want to change to.
Special Thanks to Jason. I am still not able to overlay the AutoCAD
file. Though I am now able to open it in ArcGIS but I don't know the
input projection. The output projection is State Plane NAD 83 feet.
Thanks
Taruna
Martin Alexiou Bryson
Taruna Tayal (Senior Associate)
2414 Wycliff Road Suite 101
Raleigh NC 27607
Ph: 919 881-1243 ext 209
FAX: 919 881-8081
www.mabtrans.com
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