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1500 questions
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2 answers

What's the difference between EPSG:4326 and EPSG:900913?

I am a bit confused on EPSG:4326 and EPSG:900913. I thought that they were both same. Though as I am placing my features over GMaps, which is EPSG:4326, I need to define the srsname of the WFS layer as EPSG:900913 to get it into the right place. If…
Sam007
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Learning Python programming with generic GIS goals in mind?

In your opinion, what is the best book/site to learn Python with GIS goals in mind? By "best", I mean: not very long (book) easy to understand (book/site) good practical examples (book/site) For answers specific to learning how to use the ArcPy…
Marinheiro
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49
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4 answers

Selecting features with NULL attributes in QGIS

How do I identify which features have NULL attributes in the attribute table of a shapefile? When i try [field]= 'NULL' it selects the features with value = 0 (see picture)
vascobnunes
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4 answers

What are the FOSS equivalents to these ArcGIS products?

I'm a long-term user of ESRI software, and I now have a need to use free and open source software. I've been reading about OpenLayers, PostgreSQL, PostGIS, GeoServer and MapServer but I can't find a good overview stating what each product does, why…
Stephen Lead
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48
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8 answers

How to create svg symbols that have modifiable fill color, stroke color and stroke width?

I want to create some svg symbols for Qgis using Inkscape, those symbols must have attributes as described in the question. For the last two days, I have been experimenting as instructed by Sourcepole and mimicked some symbols came bundled with qgis…
Cao Minh Tu
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48
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4 answers

What is the maximum Theoretical accuracy of GPS?

I was talking with a potential client, and they requested that we plot some points with GPS, with a maximum (or should that be minimum?) accuracy of 2 m. This is an area with no WAAS, and I was under the impression that even in the best of…
Devdatta Tengshe
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48
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4 answers

Filling column with consecutive numbers in QGIS

I make a new column in my attribute table and it has a default value (for example 0). I want to (probably using the Field Calculator) have the result that row 1 has the value 1, row 2=2 and so on, just like a numbered index. How do I do this in the…
Shepherdjo
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48
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4 answers

Creating geometry from lat/lon in table using PostGIS?

I have a table with over 800,000 records including a lat and long column. The data source did not create geometries, so I was looking around how to do this. I know there's ST_MakePoint(x,y,SRID) but all the documentation and other threads show how…
dknuth
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48
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8 answers

Exporting attribute table to Excel from QGIS

I have seen all the questions for how to export an attribute table to Excel from QGIS, and I try all the answers like copy paste, CSV (in fact this I do not know where to find), but none seems to be the right answer. Can anyone help me?
Po Po
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8 answers

Comparing QGIS and gvSIG?

Many debutant GIS users are asking me which one is better QGIS or gvSIG. Since I use Qgis and I don't have any experience in gvSIG, I aways say that both are very nice softwares and the chose is a matter of liking vanilla or chocolate. But I want to…
Pablo
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5 answers

Smoothing DEM using GRASS?

I have a SRTM DEM and I want to create a shaded relief from it. I created the shaded relief in GRASS and the result is very nice, but a little rough because the area is near flat and the DEM is 90m in resolution. What I want is to make the DEM…
Pablo
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47
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7 answers

What are Raster and Vector data in GIS and when to use?

What are raster and vector data in the GIS context? In general terms what applications, processes, or analysis are each suited for? (and not suited for!) Does anyone have some small, concise, effective pictures which convey and contrast these two…
matt wilkie
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4 answers

Reading KML file into R

I'm working with huge .kml files (up to 10 Gb) and need an efficient way to read them into R. Until now I've been converting them to shapefiles via QGIS and then back into R with readShapePoly and readOGR (the latter, by the way, is ~1000 faster…
RobinLovelace
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7 answers

Alternatives to ArcGIS Online?

I am considering purchasing ArcGIS Online for a group of users who currently use about 5% of ArcGIS capabilities (ArcMap, simple map creation and information display) about 80% of the time. Primary attractive features of ArcGIS Online for them…
r4gt4g
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47
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3 answers

Why is law of cosines more preferable than haversine when calculating distance between two latitude-longitude points?

In fact, when Sinnott published the haversine formula, computational precision was limited. Nowadays, JavaScript (and most modern computers & languages) use IEEE 754 64-bit floating-point numbers, which provide 15 significant figures of…
Afriza N. Arief
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