Images are one of the four basic object types that is supported in the Map module. An image is typically a scanned map or aerial photo in TIFF or JPEG format. Images can be imported to WMS and displayed in the background to aid in the placement of objects as they are being constructed or simply to enhance a plot. Images can also be draped or "texture mapped" onto a TIN or 2D grid.
WMS supports the following image types:
Importing an Image
The first step in using a new digital image for either background display or for texture mapping is to import the image. This can be done by either selecting the Import command in the Images menu or using the Open command in the File menu with the correct extension and opening an image file (TIFF, JPEG, or MRSID). Image files will be either georeferenced (contain coordinate information embedded) or not.
- Multiple Images
- WMS now allows more than one image at a time, however most will want to insure that each image is in the same coordinate system and that they are adjacent or it will do little good to have more than one image.
- Geo-Referenced TIFF Files
- Without embedded coordinate information about the image, it must be registered to real world coordinates when first imported. Some image files have the coordinate information embedded as part of the image and are referred to as georeferenced images. When a georeferenced image is imported to WMS, the image is automatically registered.
- Additionally, some images contain companion world files (separate text files with the coordinate information) that can be used to georeference the image. WMS automatically recognizes some world files and when they are the images are automatically georeferenced using the included information. If the world file is not recognized automatically it can still be imported from the Registration dialog.
Image Display Options
Once an image is imported, the Display Options command in the Display menu can be used to control how the image is displayed. The Image Display Options button of the Map tab brings up the Image Display Options dialog. The following display options are available:
- Draw on XY plane behind all objects
- If this option is selected, the image is drawn in the background prior to drawing any other objects. This mode is used to aid in the creation of new objects or to simply enhance a plot. The image is only displayed in plan view.
- Texture map to surface when shaded
- If this option is selected, the image is "draped" or texture mapped over the designated surface (TIN or 2D Grid). The image must be registered such that the surface lies within the domain of the image. The surface is texture mapped when the image is shaded using the Shade command.
Displays created by WMS can be captured to a TIFF image file using the Screen Capture command. These images can then be used in WMS as backgrounds, or can be used as images in other applications or report documents.
When the Screen Capture command is issued the image currently in the Graphics window is converted internally to a TIFF image. Since the real coordinates of the screen corners are already known the image is automatically registered as it is captured. These images and registration points can be exported and read back into WMS at a later time.
Image Crop Collar
Many of the images available for use in WMS are the standard USGS map series. These maps have been scanned as is and contain the information on the collar (border) of the maps. This information is okay and in fact some may want to see it, but often it is convenient to remove the collar (especially when tiling multiple images together).
The Crop Collar command, available by right-clicking on an image in the Project Explorer, can be used to automatically remove the collar from the image for display.
The Export command, available when right-clicking on the image in the Project Explorer is used to save a registered image. This command is most useful after screen capturing or cropping an image, or multiple images so that the new area can be saved as an image file.
Image And Raster Projections
Most raster data has a projection and coordinates (X, Y, and possibly Z locations of each raster cell) associated with it. The projection contains information about how to map the coordinates in the raster to actual horizontal and vertical locations on the surface of the earth.
Storing Projection Information
When loading an image or a raster, GMS, SMS, or WMS (referred to collectively as XMS) determines the horizontal projection using GDAL. This will be based on either a projection file (*.prj) or internal projection information in the raster.
If vertical projection information (vertical datum and units) is available for the image or raster, XMS reads the information and stores it internally.
If changing any projection information for a raster loaded into the a project, the XMS software saves a new raster in GeoTiff format.
Occasionally, a raster will exist that does not contain elevation data (for example, concentration values). In this case, XMS stores a metadata flag (read only by GMS, SMS, and WMS) with the raster stating that the raster does not have elevation units. When this metadata flag is found by XMS, none of the vertical values are reprojected after reading the raster.
XMS uses GeoTiffs to export rasters with horizontal and vertical projection information for use in other GIS programs.
When loading a raster, if there are no vertical units found in the raster, and the vertical values are treated as elevations, XMS prompts the user for the vertical units and exports a new raster as a GeoTiff. Also, if the vertical units do not match the horizontal units and the file being loaded is not a GeoTiff file, XMS prompts the user to confirm the vertical units and exports a new raster as a GeoTiff.
Saving Projection Information with a Raster
XMS imports rasters in a wide variety of supported formats. XMS exports rasters in either GeoTiff or Arc/Info ASCII Grid formats. No other formats are currently supported for exporting raster data.
When exporting a GeoTiff, the horizontal and vertical projection information is stored internally in the file. When saving an Arc/Info ASCII Grid file, the horizontal and vertical projection information is stored in a projection file (*.prj) and metadata file. All the necessary horizontal and vertical projection information is read into XMS (or other GIS programs such as ArcGIS) in these formats.
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