About Raster-to-Vector Conversions

Introduction

Raster and vector are the basic data structures for storing and manipulating computerized images and graphical data. All of the major GIS (geographic information systems) and CAD (computer aided design) software packages utilize both structures, allowing vector data to be viewed and analyzed on top of raster images.

Raster Images

Raster images come in the form of individual pixels. Each spatial location or resolution element has an associated pixel value, which indicates the coordinates, elevation, and any relevant attribute data, such as a color or ID number.

For GIS, CAD, or other mapping applications, raster image data is acquired by satellite or airborne sensors, such as the high resolution IKONOS, GeoEye-1, or QuickBird high resolution satellite sensors. The spatial resolution is determined by the resolution of the acquisition device, as well as the quality of the original data source. Because a raster image must have pixels for all spatial locations, the size of the represented spatial area is strictly limited. When the spatial resolution is doubled, the total size of a two-dimensional raster image increases by 400%, as the number of pixels is doubled in both X and Y dimensions. The same is true when a larger area is to be covered using the same spatial resolution.

Vector Data

Vector data comes in the form of points and lines that are geometrically and mathematically associated. The points are stored using their coordinates, and the lines are stored as a series of point pairs.

In general, vector data structures produce smaller file sizes than raster images because only point coordinates are stored. Raster images, meanwhile, require space for every pixel. This is true in cases when graphics or images have large regions, boundaries, and shapes that are the primary interest. Additionally, vector data is often considered to be more agile in a computerized environment, as it contains fewer data items and scales easily. These and other favorable attributes make the vector data structure essential to most mapping, GIS (geographic information system), and CAD (computer aided design) software packages, which might export data to vector formats such as shape files, DXF, DWG, SVC, and ASV.

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Collecting and Extracting Data for Mapping Applications

In order to complement the mapping functions that are included in GIS and other mapping and data processing software, vector data is used to facilitate the visualization of surface and subsurface features. The image acquisition process generates the initial raster image at a certain spatial resolution. The quality and resolution of the orthorectified raster image are key factors in the quality and accuracy of the vectorized data. When scanning hard copy maps, it is always recommended that clean and sharp originals are selected, scanned at a reasonable resolution, and then accurately georeferenced to the geodetic reference system and mapping projection of choice.

To ensure that the vector data, which might be extracted from digital satellite images, aerial photo mosaics, or digital map data, is free of any coordinate ambiguities outside of the project specifications, Satellite Imaging Corporation orthorectifies any satellite image data or digital aerial photography. Topographic, geological, and any other source map data is also rectified, using 75 to 100 percent of the grid ticks available on the maps.

Additionally, vector data can be merged with 3D terrain visualization mapping environments, such as those which use ESRI's ArcGIS software with the 3D Analyst module. 3D terrain flythrough scenes with vector data are produced in standard video formats, such as AVI and WMV.

Satellite Imaging Corporation uses heads-up digitizing, which is similar to manual digitizing in the manner that lines must be traced by hand. However, heads-up digitizing works directly on a computer screen, using the scanned raster image as a backdrop. As a result, the accuracy level is higher than using a digitizing tablet or automatic vector conversion because the raster images are scanned or processed at a high resolution, normally between 200 and 1600 dots per inch. With the assistance of advanced software and tools, the operator can actually work at the resolution of the raster data, and therefore digitize at a higher accuracy level. The interactive tracing method automates the line tracing process by tracing one line at a time under the guidance of the operator. This provides a significant improvement over manual heads-up digitizing in terms of digitizing accuracy and speed, especially when fully automatic raster-to-vector translation can't be applied. This is frequently the case when low image quality and complex layers are involved. The main advantages of using interactive tracing are the flexibility of tracing lines selectively and better operator control.

The final vector data can be provided in a variety of formats, such as AutoCAD, DWG/DXF, SVG, ASV, ESRI shape files, or standard ASCII X/Y/Z, and referenced to a specified survey datum and mapping projection.