Home Articles Geodata Overlay Viewer (GOVu): An innovative way to handle high-resolution imagery

Geodata Overlay Viewer (GOVu): An innovative way to handle high-resolution imagery

Satya Priya & Puneet Sharma
RMSI Private Limited, A – 7 Sector 16, Noida 201 301, India
Tel: +91 120 251 1102, 251 2101, Fax: +91 120 251 1109, 251 0963

When working with different types of data such as satellite imagery, aerial photographs, scanned topographical maps and other types of imagery, the size of image files can often become very large. For instance, it takes 3TB (3,000GB) of color imagery to take aerial photographs of a typical city at a 3-inch (almost 8cm) resolution, and it takes 1.5 TB (1,500GB) of color imagery to cover all of California at a resolution of one meter.

To address the problem RMSI has developed an unique Geodata Overlay Viewer (GOVu) application, which can be used to view imagery with a custom (encrypted) file format “.gov”. Along with the basic navigation functionalities of zooming, panning etc., the Geodata Overlay Viewer provides additional functionalities like export, merge, blend and many more such simplified functions. These functions shall be discussed and demonstrated during the conference.

How the GoVu works can be demonstrated through the following illustration. Aerial photo imagery for London covering 170sq km requires a 7.9GB data volume. Geodata Overlay Viewer or GoVu can handle the same area using 379MB of compressed data volume through a 20:1 compression ratio. This allows for not only the reduction in file sizes while compressing imagery but also gives high quality results. Such a wavelet compression of imagery offers several advantages including reduced image file size, faster access, easier distribution and the ability to work with imagery covering larger areas. Introduction
A compressed image is one that has been compressed to reduce the image file size, while still providing an almost perfect version of the original. The most effective technique is the wavelet based technique, which is a method of processing, quantizing and then encoding the image in a way that results in very high levels of compression, with an almost perfect reconstruction of the original image.

Typically a color image such as an airphoto can be compressed to less than 2% to 5% of its original size (50:1 to 20:1 compression ratio). This means that, at 20:1 compression, 10GB (10,000MB) of color imagery will compress down to 500MB, which is small enough to fit on to a single CD-ROM. Where the source image has less information, such as scanned topographic maps or airphotos over areas with less land change, higher compression rates may actually be achieved. 100:1 or more is not uncommon for compressed scanned topographic maps. Because the compressed imagery is

constructed of multi-resolution wavelet levels, one can enjoy fast roaming and zooming on the imagery – even from a comparatively slow media such as CD-ROM.

The Geodata Overlay Viewer (GoVu) is an application used to view imagery with a custom file format‚ *.gov™. Along with the basic navigational functionalities of zooming, panning etc., the Geodata Overlay Viewer provides additional functionalities like Export, Print, Lens, Telescope, Merge, Blend, Gazetteer, Index Map, Vector Overlay, Properties and Snapshot. Some of these additional functionalities are described below in detail.

Highlights of the Viewer developed:

  • Creation of the GoVu format
  • Creation of the GoVu image-maker that combines compressed ECW/MrSID raster, meta data of the image, the index map and the respective projection information
  • Demonstration of some unique functionalities

The RMSI Image Maker is an application used to make imagery in a custom file format ‘.GOV’. The technology used to develop Image viewer is built using MFC classes without any support of outside controls. However, MrSID1 and ECW2 decompression SDKs are used for image rendering. Supported Formats – ECW and MrSID encrypted inside GOV format. Inputs: MrSid or ECW file Bitmap of the Index map (Optional) (1MrSID is a registered format owned by LizardTech plc. 2ECW is a registered format owned by ER Mapper Inc.)

Inputs required for creating GoVu format:
MrSid or ECW file, Bitmap of the Index map (Optional), Meta Data of the image (Optional). Projected Top Left Coordinate and Resolution of the image in any of the following units (Meter, Kilometer, Feet, Centimeter, Inches, Miles, Yards), and Projection system of the image.

Image: Here users enters the projected Top Left Coordinate and Resolution of the image and on the bases of the same application calculates the bottom right coordinates of the image and top, left, bottom right coordinates of the index map.

Metadata: Here user can view the meta data user has attached with the file. You can attach meta data using option provided in the viewer.

Projection: User can choose the projection system in which he/she has projected the image. This is as such not used anywhere in the system it is just for the information of the user. Some of the unique functionalities:

This functionality is used to merge or overlap two images which have overlapping geo-coordinates or if they are images of same geo-positioned area.

When the user clicks on the ‚Merge™ button on the vertical toolbar, the list of images currently open appears in the list box, except for the top image which is the image to which another image is to be merged.

The images are merged in the swipe mode by default. In the swipe mode the two images are present one above the other and are separated by a red line.

The red line acts as a curtain for the second image over the first image, and as the red line is moved by the user the images are hidden and shown.

The user can hide the second image by right clicking and choosing ‚Swipe™ from Pop up menu, which will now become unchecked.

The user can also blend the two images by choosing the‚ Blend™ option

In the blend mode, the two images blend with each other.

The user can change the blend ratio of the two images using the Blend Slider Bar available on the status bar. As the user moves the pointer to the left on the Blend bar, the ratio of first image in the blend increases and vice versa.

Export is the Second Button on the Top most Toolbar of the RMSI image viewer. The Export function allows the user to create tiff, bmp and jpeg (only jpg in case of original created using ECW compression) images out of the .gov image. The user can create images of different sizes and resolutions using this function. When the user clicks on ‚Export™ button on the toolbar, a tabbed window appears on the screen.

Export Area Tab: The first tab window in the dialog box helps define the area of the image to be exported plus the resolution at which the image is to be exported.
  • The Left, Right, Top, Bottom options allow the user to specify the extents of image to be exported.
  • The Width and Height are used to specify the width and height of the image to be exported.
  • The CenterX and CenterY specifies the center of image to be exported.
  • Resolution specifies the resolution of image in m/pix at which the image is to be exported. The user can choose different resolutions from the drop-down list available.
  • The Export Area displays the area of the exported image in pixels as well as in kilometers.
  • To redefine the export area, the user should click on‚ Redefine Export Area™ button. By default the area to be exported would be the same as the current image area. On clicking this button, the export window will be hidden and the map window will be displayed to enable the user to define a new export area by drawing a rectangular box on the image.

Parameters Tab: The second tabbed window i.e. Parameters™ enables the viewer to specify the path and the path of the exported file

  • The user can change the path for saving the exported file by clicking on the‚ Export To™ button. An open dialog box will pop up where the user can browse for the path. A .goe file is created with every export, which contains the details of exported files.
  • The user can choose one of the file types from Tiff, Bmp, Jpeg and GeoTiff (only Jpeg in case of ECW compression).
  • The user also has an option of creating world and tab files (check this capability) along with the image files.
  • In case of Jpeg, the user also has an option of defining the compression level. The compression degrades the quality of the image and clicking the Preview Button can preview the effect of various compression factors.

Tiling Tab: In the third tab, the user can specify the number of tiles to be created from the selected export area and the file naming convention to be used for the exported image tiles. This feature enables exporting different parts of the image. The number of exported files will be equal to the number of tiles.

  • The user can export an area by tiling it into a maximum of 2500 individual image tiles.
  • The area of each exported image tile is specified on the top left corner of the window.
  • After cutting the image into multiple tiles, the user can select one or more tiles for eventual export. To select all tiles for export, there is an option to select all. Otherwise the user can select or deselect a tile by pressing shift and clicking on that tile.
  • The export window also displays the amount of memory needed to export the selected area and the free memory available at the bottom of the Export dialog box.
  • When the user has made his selections for export, the selected tiles are exported by clicking on the GO button.

  • When the export process commences, a dialog box appears which indicates the status of the export
  • progress, the exported file name and the estimated time left for completion.
  • If the export file which is to be created already exists at that location, then the systems asks for a confirmation to overwrite the existing file. The user also has the option to cancel the export in between by clicking on the cancel button.

Featuring GoVu:

  • Goviewer is a “viewer” for viewing compressed data formats like MrSID, ECW. It provides easy and fast access to aerial photographs. The viewer is also able to load vector data (.tab, .shp , .dxf) formats over the aerial image.
  • It has a special functionality of overlaying imagery of the same area taken at different time periods vintages and blend them. It has a Print Preview that allows the user to print customized print templates. The Quality of the data is maintained while zooming of data.
  • The most important feature of the GOViewer is its user-friendliness and ease-of-use.
  • The object-based component architecture based on n-tier architecture enables customization and extensibility of the basic system.
  • WYSWYG print: Includes creating and saving of print template, adding text and bmp to the print
  • Browser like Back and Forward features
  • Slider based zooming with beautiful graphics effects
  • Tiling and exporting of images into jpeg, tiff, bmp and geotiff formats
  • Measuring tool for calculating distance, area and perimeters etc.
  • Overlaying of shape files over the Raster image.
  • Many other features like Lens, Telescope, Gazetteer, Snapshot, Grid etc.

The Geodata Overlay Viewer is built using MFC classes without any support of outside controls. However MrSID and ECW 2 decompression SDKs are used for image rendering and libraries are used for export and vector overlays.

Supported Formats:
ECW and MrSID formats are encrypted inside .gov format. The .gov format can be created with a tool called GOMaker (included with the application). The proprietary .gov files consist of a compressed MrSID or ECW image, an index map as a bitmap (*.bmp) and metadata for the image.

Recommended System Requirements:
Operating System: Microsoft Windows 98/NT Workstation with Service Pack 6
Processor: Intel Pentium III
RAM: 256 MB
Hard Disk Space: 25 MB

Further business prospects/Conclusions:

  • Goviewer can be used as a hosted service, which can provide online access to the highest resolution, most up to date archive of Aerial photography.
  • Download AOI revealed imagery into the desktop GIS Project
  • Saving time, cost and no hassle in locating AOI image
  • No waiting time required for delivery
  • Managing Large image files/volumes

The author would like to thank Mr. Pavitra Aggarwal, Leena, Gurvinder and Farid for their wonderful contribution while developing the viewer, without their support it would had not been possible to accomplish this task.