Monday, January 10, 2011

Who has a Raster

From time to time I see users asking what is the best method to insert a raster file into Map3D or Civil 3D. There are a 4 known methods to insert a raster file into a drawing. First the standard AutoCAD insert, then there is the Map image insert command, and now with FDO there is the data connect method. Last we can use the Object Link and Embedding (OLE) method.  So what method do I suggest? My standard reply is what format and type is the raster and what do you want to do with the raster afterwards?
First lets review on what a raster file is and how they advanced over the years. A raster file contains cells or grids called pixels. The number of pixels is determined by the resolution of the raster.   A simple 300 x 300 resolution raster has 300 pixels on the X axis or horizontal and 300 on the Y axis or vertical for a total of 90,000 pixels. Each pixel has values or attributes assigned to it. One of the attribute is the location or pixel number in the file. It may be as simple as 1,1 for column 1 row 1, or 1 for the first location in the file.  Another attribute is the display value for that pixel. In the beginning the value was either 1 or 0 for on or off.  This is know as a bi-tonal raster. As technology progressed so did the values and attributes of the pixels, where today we have a color attribute and even elevation attributes along with a host of other attributes assigned to a pixel.  Just like vector objects in a dwg have attributes known as object properties like Layer, Color and custom properties such as object data, pixels have data attached to them.
Wow lets back up, did I say elevation, as in 3D? Yes I did. The technology has advance from the old DOS bmp days to create 3D raster. ESRI has been doing it for a few years now with their 3D analysis add-on for the ArcGIS programs. Some DEM files are nothing more that raster files with an elevation assigned to pixels. Same for the newer TIFF and Jpeg formats. Now when we throw multi-spectral images, know as remote sensing into this pot we not only have another a different ballgame but also a whole different ball field to play on. Multi-spectral images are made up of multiple files with each file being from a different spectrum of the light range.
The kicker to all this is not only can the pixels in a raster contain various attributes but the file extensions can all be the same. A TIFF is a TIFF. Example,  a picture we take with our Kodak Easy Share camera can have the same file extension as a image taken from a satellite orbiting the earth miles away. It is not until we open the image or raster file in the application or software that the attributes are exposed, then only part of them depending on the application used to open the file.
Using Windows Photo Galley works good to edit those family pictures from the Kodak to remove red eye, adjust the colors and print a 8 x10  photograph, but using the same application to work with a raster file that contains an elevation attributes to display as 3D will not work. It also works the other way around. Using a high end application designed for working with multi-spectral images to crop and print the Kodak Easy Share file is not only over kill but the results may not be what you expect and performance will suffer with the program stumbling over the simple attributes. To use a metaphor example of this lets say we have an acre of grass to mow with our little Briggs & Stratton powered mover. We know that if we use a high octane gasoline in an internal combustion engine the performance is increased. However if we use gasoline with 106 octane in the mower we may not get a very well cut yard. Either the engine is running too fast to get a good even cut of the grass or by the time we get half way done the engine locks up and we end up with the job half way complete. We need to use the right tool with the right material to complete the job successfully.
So if you are still reading here are my suggestions on how to add that raster file to you drawing/map.
OLE - Only use it if you working in AutoCAD LT
AutoCAD Insert – For adding your company logo to a title block or to add simple images (including renderings) as a detail to your project, where placement and scale may not be of importance.
AutoCAD Map3D Image Insert – Simple aerial images that are geo-referenced and you need them to align with your line work.
AutoCAD Map Data Connect (FDO) – Raster files that contain elevations, Multi-spectral images or high end aerials images.
Now how do you tell if the raster file is a simple aerial or a high end aerial? Most likely if you downloaded it from the Internet and it’s over 3 years old it is a good chance it’s a simple aerial unless you get the meta-data with it and it states otherwise.

No comments:

Post a Comment