4.21.04 Scanning

Mon, 02/11/2008 - 13:22
Introduction On-Screen or Heads Up Digitizing is now the most popular method of digital conversion. It is recommended that Heads-up digitizing is used. First step to be performed in Heads-up digitizing is scanning. Things to Know About Scanners There are many scanners available today. For map digitizing, two features are to be considered. Scan Size For mapping, it is best to use large scale scanners or scanners that can scan big size documents in one scanning process. There are a variety of large scale scanners; some models offering both large scale printing and scanning are available in the market. The drawback is price. Large scale scanners are expensive. There are also A4 scanners today are very common and available at a cheap price which can scan slightly larger than an A4 size documents. A4 scanners will also come in two types, flatbed and paper feed. Use only flatbed if scanning for A4 scanners. A3 scanners are also available but comes at a higher price that A4 scanners but cheaper than large scale scanners. Resolution Second thing to consider is the resolution of the scanner. Usually it is represented in DPI or dots per inch. The more ‘dots per inch’, more details are captured thus a higher resolution. High resolution scanning would also result in a larger file size since it captures more details. And it also takes longer to scan in higher resolutions. Resolution becomes a factor if we are to use maps with many details are to be captured. Preparing Maps for Scanning The accuracy of our digitize data will be dependent on how good the quality of our scanner. Thus it is important to remove any other errors that may cause discrepancies in the map. Before scanning
  1. Inspect the map to be scanned. Straighten out any folds and crumples.
  2. Check map features if they are visible and clear. If not, find better a copy of the same map if available.
  3. Check for control points* within the map. A map should have at least 4 control points. The more available control points, the better. * Control points are points on the map where exact positional location can be derived or acquired.
  4. If there are no valid control points, do a research from other maps (digitized or paper) of the same area and identify the areas which can be used as their control points.
During scanning
  1. When using small scales scanners where it is not possible to cover the whole map in one scanning, make sure to cover all possible control points for each scan portion. Allow at least 15 to 30 % overlaps in between scan portion.
  2. If a scan portion would already have at least 4 valid control points, these portions can be individually georeference. If less than 4 is available, consolidate scan portions to (using an image processing software) to produce and image having at least 4 valid control points.
  3. Adjust scanning resolution accordingly if file size will be a consideration. For black and white or single colored map, small resolution can be used as long as the output is readable.
  4. Inspect the scanned image for clarity. All control points should be visible and all features required to be digitize can be clearly distinguishable.
  5. Some scanners can adjust they scanning process for better quality by consider the surface type of the material. Some maps may be made of glossy or covered in reflective material (like laminated maps) which will tend to make contrast very bright if the light sensitivity is not adjusted. Adjust appropriately if possible. Reflective surfaces should have a lower light sensitivity setting.
  6. Save in JPEG format uncompressed.
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