GMS:Images: Difference between revisions

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Latest revision as of 20:51, 6 March 2023

An image is typically a scanned map or aerial photo in TIFF, PNG, or JPEG format. Images can be imported to GMS 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, 2D grid, 2D mesh, or the top of a 3D grid or 3D mesh.

Scanned USGS Quad Map Texture Mapped to the Top of a 3D Grid.

Image display options are changed in the Project Explorer. The options are as follows:

Visibility – The Visibility of an Image is turned off by toggling the check box next to the image in the Project Explorer.

Transparency – The Transparency of the images can be changed by right-clicking on the image folder in the Project Explorer and selecting the Transparency command.

Crop Collar – USGS quad sheet collars can be removed from an image by right-clicking on an image in the Project Explorer and selecting the Crop Collar Command.

Getting Images via Online Maps

Free imagery can be obtained from the internet via the Get Online Maps feature. A local image file can be created from an online map via the Export command.

Importing an Image

Images are read into GMS by using the File | Open command. GMS supports many different image file types.

Image Pyramids

Image pyramids can be created when an image is imported into GMS. Image pyramids are multiple resolution copies of the original image that are created and saved to speed up display. With pyramids, a lower resolution copy of the image displays quickly when drawing the entire image. When zooming in, copies of the original image resampled at finer resolutions are then drawn. This option allows for optimal performance and clear image display.

See Image Pyramids for more information.

Multiple Images

GMS allows more than one image at a time; however, when doing so make certain that each image is in the same coordinate system.

Images are displayed in the order they appear in the Project Explorer. Images that are listed above other images in the Project Explorer will be displayed above those images in the Graphics Window. The display order can be changed by clicking-and-dragging images above or below other images in the Project Explorer.

Geo-Referenced Images

Some images have the coordinate system embedded in the file. Also, some images will have a separate file called a world file (for example: a TIFF world file, *.tfw). These files are "geo-referenced." When these files are opened GMS automatically registers the image to the real world coordinate location specified. In the case where a separate world file is used, GMS will automatically find it and register the image if the world file has the same filename prefix as the image file and is in the same folder.

If the image file is not geo-referenced then register the image manually. (See Registering an Image)

When the GMS project is saved, a link to the image is saved in the project file, along with the current image registration information so that the image is re-registered to the same coordinates every time the project is opened. The original image file and world file (if one exists) are not altered.

Exporting Images

An image can be exported by right-clicking on the image in the Project Explorer and selecting the Export command. When exporting, the image file type and the resolution can be specified. Only the image which is open in GMS is exported, not the entire Graphics Window.

The entire Graphics Window can also be saved to an image file via the File | Save As command. A number of different image file types are supported such as: BMP, PNG, JPG, and TIF.

Texture Mapping Images

GMS allows draping an image over the top of a TIN, grid, or mesh. However, only the first image listed in the project explorer will be texture mapped onto the TIN, grid, or mesh. Change the order of the images in the Project Explorer by selecting the image and dragging it to a new location in the Project Explorer.

Image Properties

The Image Properties dialog can be opened by double-clicking on an image in the Project Explorer. This dialog displays information about the image, such as name, path, type, pixel size, etc.

Example of the Image Properties dialog

Displaying Images

Due to the large number of image formats and applications, XMS (GMS, SMS, and WMS) makes use of the GDAL library to classify the type of data in an image file when it is loaded into the application. XMS first classifies the image data as "Online", "Vector" or "Raster".

Online images are added to a project using the File | Add Online Image command. This command associates a URL to a web source with the image object, so XMS can always handle these images as dynamic images.

Vector images are not images at all but files that contain points, lines and/or polygons. When a GIS file (with the exception of ESRI Shapefiles and CAD DWG, DXF, or DGN files) containing vector data is read into XMS, XMS converts the vector information to an RGB image (raster) that XMS can display on the screen using a vector display pipeline. Vector data for ESRI shapefiles and CAD files is displayed as vector data without converting the information to a raster.

When opening any other image file, XMS looks for raster bands in the image. A band is a list of values for each pixel in an image. Most common types of bands include byte values, integer values, and float values. Each band may also have attributes to help indicate what the intended purpose of that data band. These attributes are not completely standard or required.

If XMS detects that an image file has raster data bands, it is processed as a raster image as described below. If no raster information is detected, XMS attempts to detect vector data in the file. If vector data is detected, XMS processes the vector data into a raster format for display. Vector image objects also include functionality to convert the vector data to map coverage data or to export them as ESRI shapefiles or other vector file formats.

Raster Images

XMS includes multiple display pipelines for raster images. These include:

Contour Raster Display Pipeline – This display pipeline is used to display contours on rasters containing floating point values:
- These rasters have one single band of any data type. There cannot be any color palette or color defined for the single band. XMS displays contours for these rasters and allows users to change the contour options.
Color Image Display Pipeline – This display pipeline is used to display color images (anything that XMS does not contour):
- If the image has a single band with a palette (such as a land use raster), XMS identifies this image as a type of index map. It converts this to an RGB image and displays the image using the palette colors.
- If the image has a single band with a grayscale attribute associated with the band, XMS converts this image to an RGB image and displays it.
- In all other cases, XMS converts the image to an RGB image based on the data in the first 3 bands and displays it. If the image has less than 3 bands, XMS uses the data in the first 1 or 2 bands as it exists and converts the image to a 3-band RGB image.

Note: if an XMS user loads an image with non-standard data, XMS will log a message indicating the type of anomaly associated with the image. These anomalies include:

XMS creates one or more bands as in the case described when the third band is generated.
XMS loads an image with bands that are of type other than “Byte”.
XMS detected extra bands that were not used in the generation of the image.

A user may desire to process the image file using external tools to clarify the intent of the data bands. This processing will normally result in a new image file, for which the data bands are clearly defined.

Vector Images

These images use the vector display pipeline in XMS. This display pipeline converts vector files that don’t have raster data to RGB images that can be displayed in XMS.

Online Images

These image use the online image display pipeline in XMS. This display pipeline converts online image services to RGB images that can be displayed in XMS.

DEMs / Rasters

See Rasters.

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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 [[File:GMS ImageProperties.png|thumb|none|450 px|Example of the ''Image Properties'' dialog]]
+{{Displaying Images}}
 == DEMs / Rasters ==
 See [[GMS:Rasters|Rasters]].
 {{Image And Raster Projections}}
 {{Navbox GMS}}

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