[Gvsig_english] determining length of lines in a polyline layer

Tue Nov 9 13:55:55 CET 2010

Hello, great!

An interesting result:

This KML:

===================================================
<kml xmlns="http://earth.google.com/kml/2.1">
<Document><name>equator degree</name>
<Region><LatLonAltBox>
<north>0.0</north><south>0.0</south>
<east>-119.0</east><west>-120.0</west>
<minAltitude>0.0</minAltitude>
<maxAltitude>0.0</maxAltitude>
</LatLonAltBox></Region>
<Placemark id="0">
<LineString><coordinates>-120,0.0 -119.0,0.0 </coordinates></LineString>
<ExtendedData></ExtendedData></Placemark>
</Document>
</kml>
===================================================

is  a simple line in the Pacific Ocean from (120W, 0N) to (119W, 0N), so its length should be:

L = 2 x pi x R / 360 = 111,319.49 m

with R = 6378137 (WGS84 ellipsoid max radius)

but Google Earth says its length is 111,195.1 m

So apparently Google Earth systematically provides a value with an error of about -0.1%

Regards,

Juan Lucas Domínguez Rubio
---
Prodevelop SL, Valencia (España)
Tlf.: 96.351.06.12 -- Fax: 96.351.09.68
http://www.prodevelop.es <http://www.prodevelop.es/> 
---

________________________________

De: gvsig_internacional-bounces at listserv.gva.es en nombre de Sal Garfi
Enviado el: mar 09/11/2010 10:25
Para: Users and Developers mailing list
Asunto: Re: [Gvsig_english] determining length of lines in a polyline layer

Hello Juan,

It appears that I should get an award for sillyness! I mistakenly selected the wrong projection when I loaded my shape file into gvSIG. I just highlighted the wrong projection in my list of recently used projections when I created a new view to work in!

Thanks so much for your efforts and help,
Sal garfi

	----- Original Message ----- 
	From: Juan Lucas Dominguez Rubio <mailto:jldominguez at prodevelop.es>  
	To: Users and Developers mailing list <mailto:gvsig_internacional at listserv.gva.es>  ; Users and Developers mailing list <mailto:gvsig_internacional at listserv.gva.es>  
	Sent: Tuesday, November 09, 2010 12:16 AM
	Subject: Re: [Gvsig_english] determining length of lines in a polyline layer

	Hello, Sal:

	Using the function perimeter() with the field calculator (not Sextante) in your shapefile, I get these values:

	d1: 31918.91 m
	d2: 3647.84 m
	d3: 12396.15 m

	while Google Earth says:

	d1: 31883 m
	d2: 3644 m
	d3: 12382 m

	The difference is again a bit more than 0.1 %

	How did you compute the fields "Length" and "Length2" ?

	Regards,
	Juan Lucas Domínguez Rubio
	---
	Prodevelop SL, Valencia (España)
	Tlf.: 96.351.06.12 -- Fax: 96.351.09.68
	http://www.prodevelop.es <http://www.prodevelop.es/> 
	---

________________________________

	De: gvsig_internacional-bounces at listserv.gva.es en nombre de Sal Garfi
	Enviado el: lun 08/11/2010 13:57
	Para: Users and Developers mailing list
	Asunto: Re: [Gvsig_english] determining length of lines in a polyline layer

	Dear Juan,

	Yes, you are right to correct me. The gvSIG distance is ca 5% less than the Google Earth distance, so I'm attaching the the original kml file and the shape file which includes two 'length' fields. The first length field was created using Sextante, while the 'Length 2' field was created by the length determination facility in gvSIG.

	You'll also notice that the kml file shows a very small part of the infamous complex of berms that have been created by Morocco in Western Sahara. I've already digitised, in Google Earth, the entirety of the berms and I now want to calculate their lengths and input them into gvSIG for further analysis. This is part of my PhD research on the war in Western Sahara.

	Best wishes,
	Sal

		----- Original Message ----- 
		From: Juan Lucas Dominguez Rubio <mailto:jldominguez at prodevelop.es>  
		To: Users and Developers mailing list <mailto:gvsig_internacional at listserv.gva.es>  ; Users and Developers mailing list <mailto:gvsig_internacional at listserv.gva.es>  
		Sent: Monday, November 08, 2010 12:18 PM
		Subject: Re: [Gvsig_english] determining length of lines in a polyline layer

		Hello, Sal.

		"95% less than" means "20 times less than", actually.

		Do you mean "5% less than"? (you expected 100 and got 95)

		I have made a test with the attached shapefile and:

		gvSIG says it's length is 50.12 m 
		Google Earth says it's length is 50.06 m

		The difference is about 0.12 %.

		Can you send your example?

		Regards,

		Juan Lucas Domínguez Rubio
		---
		Prodevelop SL, Valencia (España)
		Tlf.: 96.351.06.12 -- Fax: 96.351.09.68
		http://www.prodevelop.es <http://www.prodevelop.es/> 
		---

________________________________

		De: gvsig_internacional-bounces at listserv.gva.es en nombre de Sal Garfi
		Enviado el: lun 08/11/2010 12:31
		Para: Users and Developers mailing list
		Asunto: [Gvsig_english] determining length of lines in a polyline layer

		Dear All,

		I have recently digitised linear features in Google earth that I have converted from kml format to shape files. I wanted to tabulate the total distances of these lines in gvSIG so I used the available facility in vector line tools in Sextante. However, the distances were on average 95% less than the distances noted in the features' properties in Google Earth. I then used gvSig's own facility for determining lengths of polylines and these were the same as those determined in Sextante. I'm working in WGS84 - so what could be causing the differences? And which system is correct, Google Earth or gvSIG? 

		All the best,
		Sal Garfi

________________________________

		_______________________________________________
		Gvsig_internacional mailing list
		Gvsig_internacional at listserv.gva.es
		http://listserv.gva.es/cgi-bin/mailman/listinfo/gvsig_internacional

________________________________

	_______________________________________________
	Gvsig_internacional mailing list
	Gvsig_internacional at listserv.gva.es
	http://listserv.gva.es/cgi-bin/mailman/listinfo/gvsig_internacional

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://listserv.gva.es/pipermail/gvsig_internacional/attachments/20101109/57de6ee0/attachment.htm 