[GRASSLIST:4173] Re: Map import cluelessness

Wed Aug 11 14:01:12 EDT 2004

So as I understand it, I think you set up the mercator projection 
correctly.  One question, are you sure you want a mercator projection, 
and not a UTM (universal transverse mercator) projection?  Generally 
mercator projections are only used with large areas of the world.  With 
a UTM, you just need to input the zone number, a map can be found here.
http://www.dmap.co.uk/utmworld.htm

If you do want a mercator projection, then the typical parameters for a 
world map would be pretty much what you put
Latitude of true scale : 0
Cenral meridian : whatever is appropriate for your map
Scale factor at central meridian : 1
plural form of units : meters

This location differs from an XY location in that as you move away from 
the latitude of true scale, the vertical and horizontal scale do not 
stay equal to each other.  For example, world mercator maps really 
distort the north and south poles.  An xy location works essentially 
like Photoshop, the x axis and y axis are always the same scale.

 From what I know, all types of reprojecting like you are trying can be 
done completely within GRASS.

Does this help?

-Ian

On Aug 11, 2004, at 10:20 AM, C.S. Cornuelle wrote:

> Ian,
>
> Thanks.  I did originally attempt what you suggest, but ran up against
> a wall - of my own ignorance, probably.  :^)
>
> Namely, setting up the projection of the Mercator location does not
> seem to work for me.  To get concrete:
>
> I start with -
>
> 	D   Other Projection
>
> and choose
>
> 	Please specify projection name = merc
> 	Please specify datum name = wgs84 (no real basis for this ...)
> 	Now select Datum Transformation Parameters = well, see
>
> 	>list
> 	Number  Details
> 	---
> 	1       Used in Default wgs84 region
> 	        (PROJ.4 Params towgs84=0.000,0.000,0.000)
> 		Default 3-Parameter Transformation
>
> So I used this, as it is the only choice.
>
> Now we get to the hard part.
>
> 	Enter Latitude of True Scale [lat_ts] (0) : 0 (default for
> 	Mercator as I understand this)
>
> 	Enter Central Meridian [lon_0] (96W) : 30E (in my case)
>
> 	Enter Scale Factor at the Central Meridian [k_0]
> 	[1.0000000000]: (no idea what this is, so left it at 1)
>
> 	Enter plural form of units [meters]: (OK, it really breaks down
> 	here for me, since entering anything besides meters leads to a
> 	request for the conversion from "whatever" to meters, all of
> 	which in the context of a Mercator projection seems odd to me.)
>
> In the end, this looks just like an (x,y) location.  Clearly I am not
> understanding what is going on here.  Perhaps I need to look into
> using something like gdalwarp to translate my scanned Mercator map into
> lat-lon or?
>
>
> Thus spake Ian MacMillan (ian_macmillan at umail.ucsb.edu):
>
>> I am not sure that this will totally solve your problem, but it is
>> necessary for correct projection.  You need to import your original
>> scanned map into an XY location (without any georeferencing), then
>> project that into your desired location (either a lat-long if you have
>> those points, or a mercator if you have those) with i.group, i.target,
>> i.points, i.rectify.  This first step is necessary to originally
>> georeference your scanned image.  Then, if you want to reproject your
>> image (say from a mercator to a lat long) you need to use r.proj from
>> your final desired location (the lat-long) with the correct region
>> setting.  To find a good region setting use m.u2ll or m.ll2u to find
>> the bounding coordinates of your raster in your mercator projection.
>>
>> Hope this helps,
>> Ian
>>
>>
>>
>> On Aug 10, 2004, at 2:55 PM, C.S. Cornuelle wrote:
>>
>>> Thus spake Glynn Clements (glynn.clements at virgin.net):
>>>
>>>> C.S. Cornuelle wrote:
>>>>
>>>>> Thanks for the succinct description.  It allowed me to get as far
>>>>> as i.rectify, where this ancient (according to some web searching)
>>>>> problem arose courtesy of my mailbox:
>>>>>
>>>>> 	GIS ERROR: error while writing to temp file
>>>>>
>>>>> As per a recommendation I tried this:
>>>>>
>>>>> 	watch df -h
>>>>>
>>>>> and then some of this:
>>>>>
>>>>> GRASS:/data/grassdata > date;df /data
>>>>> Mon Aug  9 17:02:19 MDT 2004
>>>>> Filesystem           1K-blocks      Used Available Use% Mounted on
>>>>> /dev/hdb4             24590712  11836772  12753940  49% /data
>>>>> GRASS:/data/grassdata > date;df /data
>>>>> Mon Aug  9 17:02:34 MDT 2004
>>>>> Filesystem           1K-blocks      Used Available Use% Mounted on
>>>>> /dev/hdb4             24590712  11026744  13563968  45% /data
>>>>>
>>>>> While the first call of df /data probably did not catch peak disk
>>>>> usage, the difference is around 810M, not nearly large enough to
>>>>> fill the disk or the partition.
>>>>>
>>>>> Any ideas on what might be going on here?  Is there a malloc set
>>>>> in i.rectify which is causing this?  FYI this map has 7500 rows and
>>>>> 15600 columns, so it is rather large.  10 points were identified in
>>>>> i.points, and the rms value was somewhat high, nearly 100.
>>>>
>>>> On 32-bit architectures (e.g. x86), files are limited to 2Gb; it may
>>>> be that you are exceeding that limit.
>>>>
>>>> Although Linux supports files larger than 2Gb, the code which deals
>>>> with such files has to use "off_t" instead of "long", so the code
>>>> would need to be updated to allow for this.
>>>>
>>>> Actually, the GRASS code often performs offset calculations using
>>>> "int"s so, even on platforms where "long" is 64 bits, a lot of GRASS
>>>> would still have problems with files larger than 2Gb.
>>>
>>> This problem does indeed go away when the region is limited to a
>>> smaller
>>> subset of the map.
>>>
>>> However, I have another bonehead problem now.  :^)
>>>
>>> Not one but two emails are sent to me by i.rectify for this smaller
>>> region:
>>>
>>> Date: Tue, 10 Aug 2004 15:39:12 -0600
>>> GIS WARNING: WARNING lote_wholeworld_base at PERMANENT: projection don't
>>> match current settings.
>>>
>>> and then:
>>>
>>> Date: Tue, 10 Aug 2004 15:39:12 -0600
>>> Subject: i.rectify
>>> ***********************************************
>>> Rectify [lote_wholeworld_base at PERMANENT] (LOCATION LOTEWHOLE_MERC)
>>> into  [lote_wholeworld_baserec in PERMANENT] (LOCATION GLOBAL_30)
>>> complete
>>> -----------------------------------------------
>>> 600 rows, 600 cols (360000 cells) completed in 0:05
>>> 4320000.0 cells per minute
>>>
>>> I have tried this with several different regions, to no avail.  And
>>> searching about did not turn up any references to the GIS WARNING
>>> message above.
>>>
>>> The sequence of actions was to set up a Mercator location, read the
>>> scanned map file in with r.in.png, then i.group, i.target, i.points,
>>> and finally i.rectify into a lat-lon location.
>>>
>>> Any ideas?
>>>
>>> -- 
>>> Adios,
>>> Chris Cornuelle
>>> bob at xmission dot com
>>>
>>
>
> -- 
> Adios,
> Chris Cornuelle
> bob at xmission dot com
>