you're reading...
gvsig mobile, wonthurt

Quantifying gvSIG Mobile’s transformation accuracy: the case of Italy

Verto is a proprietary software package (available from the Italian IGM e-shop) to perform coordinate transformations among the various CRS used in Italy. Results provided by Verto are considered official so this software is used as a reference to estimate the accuracy of other tools.

Traspunto is a more accessible and less accurate alternative to Verto. It was developed by the Ministry for the Environment of Italy. It’s not open-source as far as I know, and is usually downloaded for free by end users like me :) According to Davide Travaglini and V. Baiocchi, M. Crespi and C. De Lorenzo, Traspunto provides submetric accuracy.

gvSIG Mobile uses three different sets of parameters (for Italy’s mainland, Sardinia and Sicily) to perform the transformation between WGS84 and the Monte Mario datum (for EPSG:3003 and EPSG:3004). The application decides which parameters must be used, depending on the input latitude and longitude:

These maps show the offset between the coordinates provided by gvSIG Mobile and the ones provided by Traspunto. The vector represented is:

[gvSIG Mobile value] minus [Traspunto value]

  • Since Traspunto is much more accurate than gvSIG Mobile, those maps are showing a good approximation of gvSIG Mobile’s true accuracy.
  • Each one of those maps is actually a composition of two different parts: for longitude < 12, it compares gvSIG Mobile and Traspunto in the transformation: EPSG:4326 to EPSG:3003, while for longitude > 12, it compares gvSIG Mobile and Traspunto in the transformation: EPSG:4326 to EPSG:3004.
  • We can see a sudden change in the argument’s variation pattern for longitude > 15. gvSIG Mobile is using the same transformation parameters for the whole peninsula, so that discontinuity has to be caused by a change in the grid nodes used by Traspunto (it probably uses nodes from a different source when longitude is larger than 15.)
  • Congratulations to Tuscany, Liguria and Sardinia (regions with highest accuracy). Pity about Apulia though (large orange area).
The modulus heatmap for Britain shown in a previous post looks like this when using the same color function I used for Italy. We can see that gvSIG Mobile provides about the same accuracy when using the traditional datums of these countries (Monte Mario and OSGB36).

Ernesto Sferlazza (from Agrigento, Sicily) has derived several interpolation grids from Traspunto in the NTv2 format (the de facto standard) which can be used in many applications like gvSIG. Downloads and details here. I actually used his grids to check that I was using Traspunto correctly.

Advertisements

Discussion

Trackbacks/Pingbacks

  1. Pingback: GIS-Lab Blog» Архив блога » Новости вокруг - 08/01/2010

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: