Projection And Spheroid Tutorial
The Projection and Spheroid Extension for ArcView 3.x, downloadable from here, is used to change the projection of the view and the spheroid the projection is based on. This can be used for almost all commonly used planetary projections and a variety of planets and satellites. It contains scripts to help with image georeferencing, lat/lon readouts, and calculating accurate distances over a sphere. This can also be used to change the longitudinal system from 180->-180 to 0->360 and to change east positive to west positive. This last feature has not been well tested and is not recommended.
Usage Hints:
- ArcView, as with this extension, can only project vector files on-the-fly which are saved in decimal degrees (in other words longitude/latitude coordinates). If the vector file is already in projected meters do not use this extension.
- In order to project your vector or feature files to images, ALL images must already be projected to your desired projection using the appropriate parameters (center longitude, center latitude, parallels,...) and be in a GIS ready image format (ie. Geotiff and other image with a world file). See the FAQ for more information.
- This is very important so we will reword it again. You can not change the projection of an image using this extension. If the image projection is different from the projection you set, the vector files will not be properly located on the image.
Tutorial:
- Setting a projection
- After the extension and spheroid.dat file is installed, start ArcView. Please see the readme file that comes with the download on instructions for installation.
- Open an ArcView project and turn on the Projection and
Spheroid extension
- Activate the Projection display, under File:Extensions:Projection and Spheroid
- Check Projection and Spheroid to activate
- Open a View
- Locate Projection and Spheroid menu
- Under View: Projection and Spheroid
- Drop down menu from Projection and Spheroid:
- Select a Projection: many projections require other parameters to be set. The extension will display the needed parameters for you.
- Select a Spheroid : The radius is automatically set by the extension. The radii are set in the spheroid.dat file. If they do not suit your needs, you may modify this file.
- Select a longitudinal system : We do not recommend changing this setting, as ArcView works best in the –180->180 east positive. Thus all of the vector files we save are stored in an Earth based positive east longitude system. See the FAQ for more information.
- Lat/lon readout
- Once a projection is set this button active. Then by clicking on the map the longitude and latitude values are returned to the lower left of your ArcView interface.
- Geodectic Distance
- This tool allows you calculate an accurate distance over the defined sphere. It only allows you calculate distances between two points.
- The standard
ArcView measure tool
calculates distances along multiple points but its' accuracy depends on
the current projection. Thus the user must be cautious when using this
tool as the distance contains the error of the defined projection.
- Image Registration (world file creation)
- This tool will help you generate a world file to georeference an image. See the FAQ for how the world file is defined. Before this tool will function, a projection must be set. Make sure it is the same projection as the image. This tool expects the latitude and longitude values for the upper left edge of the top left pixel and the map scale (pixel size in meters). The longitude must always be entered in a positive east coordinate system. Please see the MapMaker Image Registration Tutorial for more.