Background images are special vector and raster based objects that can be created to act as passive backdrop on which the rest of the network may be overlaid.
Creating a Background
Background Images are stored as files on disk and only a reference to the file and the path is stored in the .XP file. These “Image” files must be present for the background to be drawn. There is neither a practical limit to the number of background images that may be loaded into the network nor to the size of an individual image. These Background Images can be selected, deleted, moved, hidden, etc.
Three background image types are supported: These include AutoCAD .DWG, and .DXF files, GIS files such as ESRI ArcView Shape files and MapInfo files and digital photos such as .BMP, .JPG, .TIFF.
ESRI shape files and MapInfo files may also be added to an XPSWMM model as a GIS Layers.
Importing Background Images
Background data in the form of aerial photos, ESRI GIS Shape files, contour maps or subdivision layouts may be imported to facilitate the creation of the network.
If a CAD file (.dxf or .dwg) is imported as a background image it will be moved to the CAD layer. When XPSWMM encounters a file created on an earlier version with a CAD file in a background layer, the file will be moved to the CAD Layers.
Shape files may also be imported as GIS Layers.
.dxf and .dwg files should be added as CAD Layers.
To import a background select Background Images -> Add Background Image from the View menu, or choose the Add Background Image icon from the toolstrip. Alternatively, you can right mouse click on the Background Images in the Layer Control Panel and choose Add Background Image.
To load a background file select the ellipses button to navigate and find the file. Select the file or type in the name of the file and then choose open. The coordinates of the Shape file will be automatically used. If the digital image has a world file the coordinates will be obtained from them. For digital images without world files use the appropriate coordinates for the project when positioning the photo.
The outer corners (Top, Left, Bottom and Right) where the non-CAD or Shape file will be imported. The coordinates of ESRI shape files are automatically read and placed in these fields. If importing a digital image format file then the rectangle corresponds to the real world coordinate edges of your drawing in the graphics package. These coordinates determine the aspect ratio and scale of the imported background.
If a world file is located in the same folder as the image file, the coordinates will be automatically entered.
XPSWMM uses double precision coordinates with up to 20 significant figures. This enables map coordinates to be retained. Once a picture has been imported it is treated in the same manner as any other element of the network.
To edit existing background images select Background Images->Properties from the View menu or choose the Image Properties icon from the toolstrip. Alternatively, you can right mouse click on the Background Images in the Layer Control Panel and choose Properties.
New. This button will open the Add Background Image dialog.
Delete. This button will delete the highlighted background picture from the XP database.
Up. This button will move the selected background picture up one level in the list.
Down. This button will move the selected background picture up one level in the list.
Properties. If an ESRI Shape file is selected then this button will open the ESRI Shape File Attributes dialog to allow attribute encoding.
Show. This button will toggle the display or hiding of the selected background picture.
Real World Extents. The outer corners (Top, Left, Bottom and Right) where the background image file resides. For non-CAD or Shape files these coordinates determine the aspect ratio and scale of the imported background.
XP uses double precision coordinates with up to 20 significant figures. This enables map coordinates to be retained. Once a picture has been imported it is treated in the same manner as any other element of the network.
ESRI Shape File Attributes
Shape Filename. The name of the Shape file that is being Encoded.
Dbase Filename. This is the name of the database file that contains the attributes being encoded.
Encode Attributes. This is a check box to turn on/off the encoding of the selected attribute.
Encoding Field. This combo box allows the user to select the attribute in the database file for encoding.
Color. This is a column of colors assigned to the field value of the same row. Selecting the on the cell brings up the Windows Color palette shown below.
Field Values. This is a column of the selected attributes from the database file. To the right is the color assigned to that shape file element.
Many times TIF, JPG and BMP image files are accompanied by a “world” file that provides basic georeferencing information (but not all information) about the image file. XPSWMM recognizes files with the following extensions as world files:
.tfw (when a .tif file is opened)
.jgw, .jpegw or .jpw (when a .jpg file is opened)
bpw (when a .bmp file is opened)
The world file provides basic (but not complete) georeferencing information that allows XPSWMM to automatically fill in the coordinates of the Destination Rectangle in the Image Properties dialog.
A World file is a simple ASCII text file that contains the X and Y pixel size, the rotation, and the registration coordinate of the center of the upper-left corner pixel in the image. Following is a sample World file with descriptions.
X pixel size
Rotation term for row
Rotation term for column
Y pixel size, negated to represent upper-left of image
X coordinate of center of upper-left pixel
Y coordinate of center of upper-left pixel
The pixel size and registration coordinates are expressed in units of meters or feet. (The Y pixel size is usually a negative value to account for the differences between the origin of image and non-image data.
World files do not contain some georeferencing information including:
Geodetic Datum. A spheroid and tie point, this defines the local variation in the shape of the earth's surface in that area of the world
Map Projection. The model used to translate spherical earth coordinates onto a flat 2-D surface (the map). Common projections include Universal Transverse Mercator (UTM), Transverse Mercator, Lambert Conformal Conic, and many others.
Units. The measurement units used for coordinates in the selected map projection.
CAD File as Background Image
XPSWMM can import CAD drawing files (.dwg) as bakcground images. The imported file will appear under the CAD files section in the Layer Control Panel.
Editing Background Transparency
To change the transparency of a background image:
- Select and right-click the Backgound Images layer.
- Select the Properties to launch the Background Image Properties dialog, and click the Properties button.
- In the Add Background Image dialog, move the Transparency slider to desired level of transparency.
- Click OK on both dialogs to accept the change. The background image will now show the transparency change.