On-screen digitizing versus digitizer tablets
MiraMon does not support digitizer tablets, but do not panic. Let
us explain. We have been in GIS and Remote Sensing during the last
11 years and, believe us, after trying on-screen digitizing,
you never go back to tablets.
MiraMon provides all the functionalities needed for on-screen
digitizing: georeferencing (including orthophoto generation),
mosaicking, digitizing (including using entitites from other layers,
copying them or connecting to them) and true topology building.
- 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 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.
- 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.
- 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).
- 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).
- 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.
Other related notes: