[postgis-users] drawing nautical data on projection 4326[solution?]

[Dave]

On 29/04/2011 13:22, Morin, Marc-André wrote:
Hi Marc,

Thanks for the advice,  I had choosen to use 4326 mainly because its the 
default in postgis for so many calls.

I shall just convert my data to 900913/3857 and forget having to convert 
anything on the fly.
> Hi Dave,
> You are certainly not on the right track, because the SRS EPSG:4326 is 
> not a projection system, but more a datum (WGS84), based on an 
> ellipsoid (GRS80).  In a nutshell, the 4326 srs is a "spherical" 
> geographic system and that is why the unit of measurement is in 
> degree.  So when you store your data in EPSG:4326, that means your 
> data will have to be stored in latitude and longitude, and you cannot 
> represent lat-long coordinates directly on a map, unless these 
> coordinates are projected in a cartesian (X and Y) coordinate system, 
> like the Google Mercator projection (EPSG:900913).
> That said, you specified in your first mail that your were working 
> with "lines" measured in nautilcal miles, within a grid graduated for 
> 0 to 100.  That means that you are already in a "projected" coordinate 
> system.   Now, we have to figure out what is this coordinate 
> system. Maybe I am wrong, but your 0-100 grid seems to represent a 
> cartesian coordinate system in GRAD (not in degree)...
> Could you check and confirm that at first?  Also, a screenshot of your 
> map would be helpful...
> Regards,
> Marc-André
>
> ------------------------------------------------------------------------
> *De :* postgis-users-bounces at postgis.refractions.net 
> [mailto:postgis-users-bounces at postgis.refractions.net] *De la part de* 
> Dave
> *Envoyé :* 29 avril 2011 04:05
> *À :* PostGIS Users Discussion
> *Objet :* Re: [postgis-users] drawing nautical data on projection 
> 4326[solution?]
>
> I think I have a solution to this issue
>
> I am going to convert my point origin to meter based projection (eg 
> google 900913), create my line data, then transform it back to the 
> 4326 projection,  in this way the scale factors calculations should be 
> done by the proj library.
>
> Does this sound like a decent solution?
>
> On 28/04/2011 10:48, Dave wrote:
>> I have some line  data measured in nautical miles.
>>
>> I want to draw these points on a map using the 4326 projection, the 
>> srs entry for this projection gives the unit of measurement as degree.
>>
>> A nautical mile is one minture of an arc of lattitude ie 0-60, the 
>> projection is measured in 0-100.
>>
>> So does this mean that when I want to draw a line measured in 
>> nautical miles,  I have map 0-60 on to 0-100 grid , ie I have to 
>> apply  0.6 factor ?
>>
>> So a line  starting at lat 1.2 and ending at  lat 2.7 becomes a line 
>> 1.12  to  2.42 ?
>>
>> regards
>>
>>
>> Dave.
>>
>>
>>
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>
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