[Proj] Converting to WGS84 Latitude/Longitude

[Mikael Rittri]

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<mailto: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<mailto:vbubbly21 at gmail.com>>:


On Fri, Oct 24, 2014 at 3:50 PM, Mikael Rittri <Mikael.Rittri at carmenta.com<mailto: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> [mailto: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<mailto: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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