[Proj] Converting to WGS84 Latitude/Longitude
Artur Bercik
vbubbly21 at gmail.com
Fri Oct 24 07:53:33 EST 2014
Dear Mikael Rittri
Thank you very much for your well-explained answer.
I have adopted it.
On Fri, Oct 24, 2014 at 9:45 PM, Mikael Rittri <Mikael.Rittri at carmenta.com>
wrote:
> Artur,
>
>
>
> The reason why the geodetic datum is not documented for your dataset,
>
> can be that it is not very important. The resolution of the dataset was 1
> km,
>
> but the usual mismatch between two geodetic datums would seldom be
>
> more than 500 m.
>
>
>
> For example, let’s assume that your data was given in Pulkovo 1942.
>
> If I am not mistaken, one way to transform your coordinates would in
>
> that case be to use cs2cs like this:
>
>
>
> >cs2cs +proj=longlat +ellips=krass
> +towgs84=23.57,-140.95,-79.8,0,0.35,0.79,0.22 +to +proj=longlat
> +datum=WGS84 -f "%.5f"
>
> 117.69043 50.89524
>
> 117.69118 50.89578 -146.13626
>
>
>
> where I used the datum shift EPSG:5044, “Pulkovo 1942 to WGS 84 (20)”,
>
> valid in Russia with 3 m accuracy.
>
>
>
> If this was the correct transformation, but you forgot to do it, the
> difference
>
> would be only 80 meters, which could be insignificant when the data
> resolution
>
> is 1 km.
>
>
>
> But of course, that would depend on the usage of the dataset.
>
>
>
> Well – if this is a NASA product, I would bet that they use WGS84.
>
>
>
> Regards,
>
> Mikael
>
>
> ------------------------------
>
> *From:* proj-bounces at lists.maptools.org [mailto:
> proj-bounces at lists.maptools.org] *On Behalf Of *Artur Bercik
> *Sent:* Friday, October 24, 2014 12:48 PM
> *To:* PROJ.4 and general Projections Discussions
> *Subject:* Re: [Proj] Converting to WGS84 Latitude/Longitude
>
>
>
>
>
>
>
> On Fri, Oct 24, 2014 at 7:30 PM, Charles Clercq <charles.clercq at gmail.com>
> wrote:
>
> Dear all,
>
>
>
> I found a pdf document:
> http://www.yale.edu/ceo/Documentation/MODIS_data.pdf
>
>
>
> which state: "The data are provided in the HDF-EOS format. MODIS data at
> version 4 and above use
>
> the Sinusoidal projection with the WGS84 datum. A very small sample of
> common
>
> products are listed below."
>
>
>
> Hope it helps
>
>
>
> Thank you Charles. However, the statement you have found is for land
> products, and I am using atmospheric products. In this case, the data are
> provided in geographic coordinate system. The pair of lats and lons I have
> read are in geographic coordinate system, but do not know the datum.
>
>
>
>
>
> 2014-10-24 9:30 GMT+02:00 Artur Bercik <vbubbly21 at gmail.com>:
>
>
>
>
>
> On Fri, Oct 24, 2014 at 3:50 PM, Mikael Rittri <Mikael.Rittri at carmenta.com>
> wrote:
>
> Hi Artur.
>
>
>
> It could be the same thing. “Geodetic” latitude is the same as ordinary,
>
> everyday latitude. There are some other flavours of latitude as well,
>
> auxiliary latitudes that are mainly used to simplify some kinds of
>
> computations, but “geodetic” is the ordinary kind.
>
>
>
> For longitudes, there is only one flavour, but the zero meridian is
>
> not always Greenwich.
>
>
>
> So the coordinates you mention could be in WGS84 Lat/Long already.
>
> But it is not certain, since they could be expressed in some other
>
> geodetic datum that uses a different ellipsoid.
>
>
>
> Do you have any documentation about the geodetic datum of your
> coordinates (like Pulkovo 1942, Pulkovo 1995, PZ 1990, or WGS84),
>
> or about the ellipsoid used?
>
>
>
> Dear Mikael Rittri
>
> Thanks for your answer.
> Actually, I could not understand which datum or ellepsoid was used. The
> following documents says only geodetic latitude and longitude:
>
> http://modaps.nascom.nasa.gov/services/about/products/MOD03.html
>
> I have read the above data product; and retrieved the pairs of geodetic
> lats and lons as the document says, but could not figure out if I should
> convert them into WGS84 or not. I have to compare the data using above
> product with other data which are in WGS84 lats and lons.
>
>
>
> Best regards,
>
>
>
> Mikael Rittri
>
> Carmenta
>
> Sweden
>
> http://www.carmenta.com
>
>
> ------------------------------
>
> *From:* proj-bounces at lists.maptools.org [mailto:
> proj-bounces at lists.maptools.org] *On Behalf Of *Artur Bercik
> *Sent:* Friday, October 24, 2014 8:27 AM
> *To:* proj at lists.maptools.org
> *Subject:* [Proj] (no subject)
>
>
>
> Dear Proj list members:
>
> Being a very new in this field, could someone answer me about the
> following problem:
>
> I have a pair of Geodetic Latitude and Geodetic Longitude (50.89524,
> 117.69043).
> How can I convert it into Latitude and Longitude with WGS84 Ellepsoid?
>
> Thanks in the advance.
>
> Artur
>
>
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