On-screen digitizing versus digitizer tablets

Why?

Because:

1. It is more comfortable:
   • You have a single place where to look at: the screen. Forget about moving your head (or even your body in large tablets having far monitors) from the screen to the paper and back to the screen.
   • The control of which elements have been already digitized is very easy because it is totally visual: simply look at your screen, where you see the original map and, at the same place, the elements already digitized. On the oher hand, controlling it on the tablet is uncomfortable, because you must to look at the screen (where only the digitized elements appear) as well as at the the table (where the reference map is).
   • The task is done in your usual desk, independently of the size of the map to digitize, not being necessary to be standing up when using large tablets.
2. It is more accurate:
   • You do not need to calibrate the tablet different times (having slightly different metrics). Instead, you concentrate on the task of georeferencing the scanned document only once, and it is done. Moreover, because you can zoom in at the control points used for georeferencing, it is much more accurate (the thick crosses usually used as control points become wide and it is easy to mark the exact central point). Our experience is that usually you obtain an RMS error below 0.2 mm (for example, below 10 m on a 1:50000 map), i.e., you do not introduce extra error due to the table calibration, because the accuracy of the georeferenced scanned document is normally as good as the original paper map. These figures are rarely obtained when using sticky tape to fix the map to a tablet and trying to click on the middle of a very thick cross.
   • The georeferencing algorithm is usually better, because it uses more points and more adequate equations. In many softwares used for digitizing, only two points are used for tablet calibration, and this does not allow for correcting distorsions of different magnitude along the X and Y axis (quite common in maps hung up during long time); even if the X and Y scaling factor is exactly the same, it is highly dangerous because you have zero degrees of freedom, so if you do some mistake when digitizing the control points or when typing the corresponding map coordinates, you never become aware of it (the RMS is always falsely 0).
   • During the digitizing process you use a zoom level being moderately higher than the original one (for example x4 or x5 with respect to the original scale). This causes you to work as if you were digitizing using a powerful magnifying glass over the paper map, so the result is very close to it, and of course always more faithful than the process done when bent over the tablet. Moreover, you can do extra zoom in the more complex areas, which is hardly feasible over the tablet. In short, you are more confident about the degree of coincidence of the original elements with respect to those you digitize.
   • During the digitizing process you can use previously existing cartographic layers and to copy them or to connect to them. This is specially useful when updating a land use map or when recreating old land uses from old aerial photographs and a more recent digital land use map: the preexisting elements are very helpful and often save digitizing time because they can simply be reused. When using a tablet, when your eyes look at the map (upon the table) you do not see if there are other elements to use, connect to, etc, at most they are on the screen. When using on-screen digitizing it is straighforward because both the scanned and georeferenced map or orthoimage are on the screen as the other cartographic elements you wish to use.
   • You can change the cartographic projection of the scanned and georeferenced document before starting the digitizing process. It is specially interesting when you are digitizing a map that uses a different cartographic projection than the usual in your current project. Perhaps you think "I can reproject after digitizing on my tablet", but in this causes you hardly will use other preexisting digital layers to help the digitizing process.
   • As pointed out before, it is much more difficult to forget digitizing some entity, and it is practically impossible that you digitize twice the same element, both problems being possible when using digitizer tablets because you are not looking at the paper and at the digitized elements at the same time.
3. It is faster:
   • You do not need to recalibrate your tablet at regular time intervals, or each time you move the paper (because you may have moved it slightly after hours of work) or, -as in many softwares- after you turn off/on your computer and/or tablet or each time after you stick up the map on the table. Moreover, do not forget that most papers dilate or simply move after some hours of being tightly fixed on the table.
   • Although you may think that digitizing at zoom levels being moderately higher than the original one (for example x4 or x5 with respect to the original scale) is slower, it is not, because you can see more clearly the line you are following (it has a width of several pixels) and you can digitize faster or at least not slowly. Of course if you prefer going still faster by working at a zoom level similar to the original, you can zoom out, and draw 'from far' (as when being over the tablet).
4. You get more information:
   • Indeed, once you have the georeferenced raster ready for digitizing, you also have two extra valuables:
     • A digital copy of the paper document, very convenient for backup purpouses (specially interesting on ancient maps), for network or CD publishing (provided, of course, you have the appropriate copyright permissions) or simply to share it with your colleagues (the document fixed upon the table cannot be shared).
     • The original paper map you can always consult through MiraMon, overlapped with other digital maps if desired because it is a georeferenced digital document. This is useful whenever you want to check the original map:
       • Because it contains some elements not digitized yet (as toponyms).
       • Because it contains ancillary information external to the map face.
       • Because it is a paper orthophoto or satellite image and you digitized your photointerpretation, but you wish to check the original photographic image again.
       • Because you suspect that an error occurred during the digitizing process (some attribute looks very strange).
5. It is less expensive:
   • With MiraMon, you only need your PC and the scanned document, so you can save the tablet cost. Of course you can think you need a scanner, but for small documents (up to A3 in size) scanners are affordable and perhaps already available in your organization. On the other hand, the scanner is occupied only during the scanning process, which is very short with respect to the time devoted to digitizing, therefore several people can share the scanner (in front to the fact that you need a tablet for each person digitizing over a tablet).
     For large documents you can mosaic the results of scanned and georeferenced pieces (MiraMon is able to do that) or you can buy a larger scanner, or you can scan the whole map in some establishment having a large format scanner. This last strategy is the most widely used; for example, in Barcelona, scanning an A0 document costs about 7.50 Euro (about 6,96 US$). So only after scanning many (hundreds or a few thousands) maps you reach the cost of a large scanner or a large tablet.
   • You save an important place in your room or table.

• Digitizing and topology building in MiraMon versus other softwares