# [Proj] Equal Arc Projection, GeoTIFF, and GDAL/Proj4

Charles Karney charles.karney at sri.com
Sun Mar 2 06:09:10 EST 2014

```Got latitude and longitude switched!!  I should have said

Products like CIB05 are equal arc with nominal resolution 5m x 5m.  What
this means is that the latitude resolution is fixed at 0.000044956841432
deg (about 5m) and the longitude resolution is fixed in each latitude
"zone" to give roughly the same ground spacing.

On 03/02/2014 06:04 AM, Charles Karney wrote:
> I should have added.  See
>
> http://earth-info.nga.mil/publications/specs/printed/89041/89041.pdf
>
> for the specifications for CIB imagery data.
>
> On 03/02/2014 06:02 AM, Charles Karney wrote:
>> Products like CIB05 are equal arc with nominal resolution 5m x 5m.  What
>> this means is that the longitude resolution is fixed at
>> 0.000044956841432 deg (about 5m) and the latitude resolution is fixed in
>> each latitude "zone" to give a roughly the same ground spacing.  For
>> example, 004VB41A.I23 is centered at 52d6'3.68"N, its resolution, is
>>
>>    Pixel Size = (0.000073242187500,-0.000044956841432)
>>
>> or about 5.015m x 5.002m.  The size of a CIB05 tile is about 7.5 km x
>> 7.5km so this ground resolution is nearly constant across the image.
>>
>> You can obviously do something similar if the requested resolution is
>> 6m x 6m.
>>
>>    --Charles
>>
>> (On 03/02/2014 04:30 AM, Frederik Ramm wrote:
>>> Hi,
>>>
>>> On 02.03.2014 02:08, Charles Karney wrote:
>>>> Equal arc just means equal arc increments in latitude and
>>>> longitude.  In
>>>> other words the projection is just ordinary longitude and latitude.
>>>
>>> Duh. That sounds a good deal simpler than I thought. Thank you.
>>>
>>> Am I then right in saying that a request like "I want an image in Equal
>>> Arc projection with a pixel size of 6 metres by 6 metres" does not make
>>> a lot of sense, since the width of a pixel will be constant in arc
>>> increments, and therefore not constant in metres?
>>>
>>> Bye
>>> Frederik
>>>
>>
>

```