[GRASSLIST:68] The newbie faq

[Antonios Christofides]

[Note on crossposting: This goes both to grass-doc and grasslist.
Reply-To has been set to grass-doc, so discussion continues there.]

Hi, everyone.

I wanted to go trekking at a nearby area, and I got so pissed off about
there not being a decent map on the market that I decided to make one
myself :-) I believe I can get a DEM and other digital data, so all I
needed was free GIS software. It didn't take me long to find GRASS. I
started playing with it about a week ago.

Now while the reference documentation is rather good, the tutorial has a
long way to go, and there are numerous problems I faced, as a newbie,
trying to make the thing work. With the mailing list server struck by
thunder, I was lucky to meet a guy nicked geoalf on the new #grass irc
channel at freenode.net, who helped me take my first steps.

Under encouragement by geoalf, I wrote down all my questions in order to
turn them into a faq later, and I've even written a few answers. I
attach the current pre-pre-pre-draft at the end of this message.  PLEASE
NOTE: If you want to make comments on the draft, you are welcome, but
you'd better wait. It's really pre-pre-pre-draft, and it needs really
much work before being submitted for comments. I'm only attaching it in
order to show you that I already have something.

It is not my intent to make something like "Anthony's GRASS faq" - that
would be silly. Besides, I can't: I've only been using GRASS for a week,
and I'm not expert enough. It's useful that I'm a newbie, and thus I can
pose the questions from a newbie's point of view, but for many of the
answers the gurus will have to help. So either it's going to be
official, or I'm not making it. If there is already any official faqing
work of which I'm unaware (I doubt so, there would be traces in the
grass-doc archives), I'll probably volunteer to contribute; if not, I'm
proposing mine as a candidate.

So tell me what you think, and if you think it's a good idea then we'll
have to discuss the license, the faq/documentation tool, and the
writing/commenting procedure.



GRASS newbie FAQ pre-pre-pre-draft
==================================

Concepts
--------

What are all these projection systems? Which one should I use?

    When we create maps, we project the surface of the earth, which is
    curved, to a plane. Even if the earth were a sphere, this wouldn't
    be a straightforward process; there is no universal way to make this
    projection. Some projections may alter the lengths but keep the
    areas, and vice versa. No projection can be used to faithfully
    represent length, area, direction, and other qualities altogether.
    As if this was not enough, the earth is not a sphere; it's not even
    an ellipsoid; we have to make assumptions concerning its shape.

    Depending on what you want to do, you have to use a different
    projection system. A map can be made to accurately represent areas,
    distances, or directions, but not two of these at the same time. A
    map can also be made so that a straight line is the shortest
    distance between two points. Some maps may be attempting to
    compromise more than one thing.  Small areas, for example 100 km
    across, are approximating a plane, but a time will come when the
    small area will have to be put together with an adjacent small area,
    and there may be trouble doing so.

    You may be tempted to use Latitude-Longitude, because it is accurate
    worldwide, but it is really much easier to work with other systems,
    because you will be using units of measurement such as meters or
    miles.  The most widely used projection system is UTM, but you also
    need to know the ellipsoid name (this is the approximation you are
    going to use for the earth's shape) and the UTM zone - there are 60
    of them. In Greece, for example, we usually use the GRS80 ellipsoid
    and zone 24. You probably want to use the same projection system as
    that used by your input data, such as your digitized/scanned map.

    [ OK, there's lots of speculation in the last paragraph, don't hit
    me if I've got things completely wrong. That paragraph is more of an
    example than an actual answer. ]

What is a location, a mapset, and a map?

What is a region?

How do I organize my work in layers?

    GRASS does not have layers; instead, it has maps. You put each layer of
    information in a different map.

    The information in a map can be organized in different categories, so you
    probably want to put all roads in one map and use different categories to
    depict motorways, highways, and so on; you probably want to put all water
    streams in another map and use different categories to depict large rivers,
    small rivers, perennial streams, intermittent streams, etc.
    
What is a site?

    Vector maps contain lines, areas, and points; we use the term "site"
    for points. A mountain top and a spring are sites. A village can be
    a site or an area, depending on the scale of your map.

What is a category number, value, label, description, and attribute?

What is a Digital Elevation Model? What is a Digital Terrain Model?

What resolution should I choose?


Processing raster maps
----------------------

How do I combine my three rgb raster files into one?

    Use r.composite.


Processing vector maps
----------------------

How do I digitize a scanned map?

How do I remove a line from a vector map?

    With v.digit, Edit menu.

What do I do with contours going over the edge of the map?

How do I do something with the area outside a polygon?

How do I get v.surf.rst to process only the land and not the sea?

    You need to prepare two maps: contours and land. The contours map
    has the contours as lines, and the land map has the coastline as
    area edge; you need to label the land areas as such, and leave the
    sea unlabeled.  You must convert the land map to raster with
    v.to.rast.

    Then, use
      v.surf.rst input=your_contours_map maskmap=your_raster_land_map

How do I copy part of a vector map to another vector map?

    Copy the entire map using g.copy, and then delete the information
    you don't want from the new map (using v.digit).

    If you want to merge a part of a vector map A into an existing
    vector map B, first use g.copy to create a copy C of A, then delete
    the information you don't want from C, then use v.patch to merge B
    and C.

How do I create a coastline/sea?

    The coastline has two properties: first, it is a contour with
    elevation zero, and as such it is a line. Second, it is the edge of
    land, and as such it is an area edge.

    You should thus include the coastline in two maps: in your contour
    map, where it should be a line with elevation zero, and in your land
    map, where it should be an area edge thus marking which areas are
    land and which are sea.

    [++ information on how to specify elevation zero]
    [++ should I include the coastline in the streams map? ]

Do I want to compress the atts file?

How can I draw thicker lines?

    You can't specify line thickness on the display monitors. You can
    only do so when you prepare a map using ps.map.

Are contours lines or area edges?

    Lines.

How can I find out the category and label of a line/area/site?

    Use v.what, preferably with the -i option.


Creating maps
-------------

I want ps.map to print each category of a map with different characteristics.

ps.map does not draw sites.

How do I draw the sea with ps.map (background color?)

How do I create a shaded relief map?

How do I select the colors to use for various elevations?


General
------

How do I copy/rename a mapset?

s.menu is not installed.