Tagged Questions

Measure of quantity of a two-dimensional surface

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13
votes
1answer
738 views

Is it possible to gray out a Google Maps map except for some area?

What i am looking for is a way to emphasize a certain area on a Google Maps map by graying out the rest of the world. I heard about some solution of using 2 polygons of which 1 is visible and the ...
11
votes
5answers
3k views

How can I measure area from geographic coordinates?

If I have polygons in geographic coordinates (WGS84), how do I measure the total area each takes on the surface of the earth, taking into account the curvature of the earth?
6
votes
3answers
307 views

How do I get the area of a WGS84 polygon in square meters?

The following gives me the area of the polygon in square degrees: SELECT ST_Area(ST_GeomFromText('POLYGON((871325.790874952 6105405.3261047,871418.748307692 6105359.72944624,871346.22022442 ...
6
votes
1answer
89 views

Dividing the world space to geographic sub-space

I am trying to divide the world space to geographic sub-space from "general" to "specific" locations and without using latitude and longitude information. That is, I am trying to think about the world ...
6
votes
1answer
182 views

QGIS define hexagon area around mapped points

I am using QGIS to map a number of radio antennas (for mobile network). Once I have the layer of the sites (done), I would like to assign to each site a coverage area (envelope). I could achieve that ...
6
votes
2answers
270 views

calculate area of vegetations in a raster file

How do I calculate area of several vegetation types in a raster (.img file type) image. They are color coded. Am using QGIS (1.8.0) and ERDAS 9.2.
6
votes
2answers
247 views

How to get correct area calculation using Google Mercator?

i use QGIS with openlayers-plugin and i´ve loaded google satellite as a layer. what i want to do is making a shape-layer (polygon) with the goal to get the area values of the polygons. i´ve activated ...
6
votes
1answer
336 views

How to calculate te area between contour lines on a DEM with Qgis 1.8?

Does anyone know how to calculate the area in km² for each altitudinal band (between 100 and 200, 200 and 300...) based on a DEM? I added the contour lines with a distance of 100 metres but could not ...
6
votes
2answers
1k views

Openlayers geometry: how to convert area from degrees to square kilometres?

I get a multypolygon from my web service and want to calculate the area of that polygon. I was trying to use this piece of code: var poly = new OpenLayers.Geometry.MultiPolygon(poligon); var area = ...
6
votes
1answer
124 views

How do I represent the whole Earth as a Polygon?

This is related to determining the interior of a Polygon. My difficulty is that if I specify a Polygon covering the Earth going from East to West, it will still have a boundary along the ...
6
votes
2answers
151 views

How do different GIS systems determine a Polygon's interior?

I'm interested in understanding how real-world GIS systems and their data encode Polygons. Specifically, how do they resolve the ambiguity of a Polygon's interior on a sphere? Background: in 2D, ...
5
votes
1answer
235 views

Geometry Calculation

Wanted to ask about accuracy of areas calculated by GIS compared to actual ones on ground. Because I have data of my country without areas for the districts within Uganda. Are the areas obtained using ...
5
votes
2answers
1k views

Convert Long and Lat Co-ordinates to Square Miles

I have a set of Long and Lat co-ordinates making up a five point polygon which I have converted in excel using the following formula: ...
5
votes
1answer
448 views

How to measure area correctly?

I have now used my entire evening, but I can't figure out how to make areal calculations (eg. in km2). I have tried the 'field calculator', I have tried the 'measure area' function on the "attributes" ...
5
votes
1answer
116 views

Projections appropriate for global analysis that preserve area and that preserve distance (but not necessarily in the same projection)

I am looking for a projection (or a set of projections) that decently preserve distances so that I can buffer every country in the world and have a similar level of minimal distortion for each. I ...
5
votes
2answers
552 views

How to split an irregular polygon into equal areas?

I am a surveyor and use QGIS for maintaining soil test records on civil projects and am constantly provided with test lot area or number of lots required. These are more often than not irregular in ...
5
votes
2answers
155 views

PostGIS - area-weighted calculation on an intersection

I'm trying to learn postgis sql by performing some familiar operations from ArcGIS. Here, I want to perform an area-weighted calculation of certain populations on the intersection of a set of parcels ...
5
votes
0answers
596 views

How to split polygons without ArcView?

I'm trying to split polygons into equal areas. While fishnet sort of works; the areas I'm trying to split don't touch; they're national parks. Fishnet spans the whole area, not just the polygons in ...
4
votes
2answers
2k views

How to calculate polygon areas and perimeters?

If I load a polygon layer and make a new column, how do I fill that column with the 1) area of each polygon and another column with 2) the length of the perimeter of each polygon?
4
votes
2answers
291 views

How to calculate area of 1 x 1 degree cells in a raster

I have a simple array (not geo-referenced in any way) in Matlab (which is not a language I know a huge amount about...so I can export it to other languages if needed) which has the dimensions 180 x ...
4
votes
4answers
207 views

How to compute geodesic area in GeoDjango?

I have some polygons which can be anywhere in the world. I would like to compute their areas. The GeoDjango docs for GEOSGeometry.area don't specify whether it is geometric or geodesic area, but ...
4
votes
2answers
246 views

Total newbie. Creating map from aerial photo of golf course in QGIS

I have a aerial photo in tif with a tfw file of the golf course that I work at. I would like to create vector data of different features on the course such as greens fairways tees, etc. to calculate ...
4
votes
2answers
405 views

Why does ST_Area(g.geom) + ST_Area(p.geom) - ST_Area(ST_Union(g.geom,p.geom)) produce negative result when using ST_Overlaps(g.geom, p.geom) = true?

I have two set of shape data. Let's say file gov.shp contains list of post codes and their boundaries. File post.shp contains list of government area name and their boundaries. I want to find out the ...
4
votes
1answer
127 views

The number of digit of decimal point when you calculate area of polygon in QGIS1.8.0

I would like to change the number of digit of decimal point when I calculate area. For example Now: 32.456677 Ideal: 32.46 I try to adjust "output field width" and "Precision" These don't work well. ...
4
votes
2answers
728 views

How to find the area of raster values within polygon zones?

I'm using ArcGIS 9.3 and I just can't seem to get this right - I've tried tabulate areas, zonal stats, converting to polygons, etc. I'm missing something somewhere. I have a USGS impervious surface ...
3
votes
4answers
3k views

How to recalc WGS84 Shape_Area field to square miles in ArcGIS 10

What are the units of the Shape_Area field in a wgs84 feature class? Is it decimal degree squared? I would like to convert that to something more meaningful like square miles without reprojecting the ...
3
votes
1answer
130 views

km2 or sqkm - which is more correct?

What is more correct, km2 or sqkm? I'd prefer to use km with a superscript 2, but it's not always possible to format text that way so I was just wondering if there is a convention that should be ...
3
votes
2answers
67 views

How to calculate the shared area between a raster and a shape file in ArcGIS 10?

I have a raster file with 5 classifications developed from habitat modelling algorithms. I'd like to know how much of 2 classes of the raster is covered by the network of protected areas (Shape file). ...
3
votes
3answers
187 views

Auto-calculate field (area or formula)

I'm wondering if anyone has any insight into the potential to auto-calculate the value of a field, either area or distance (ideally) or a formula (based on the values of say 2 other fields, or an ...
3
votes
1answer
298 views

How can I get my Qgis area calculations in metres rather than degrees? My shapefiles are projected using WGS 1984 UTM fuseau 35 Sud

I am using Qgis 1.7.4 to calculate the area of polygons. My shapefiles have been projected using the coordinate system WGS 1984 UTM fuseau 35 Sud, but my area calculations are still displayed in ...
3
votes
3answers
8k views

Calculate projected area of polygons with field calculator in ArcMap 10

I've got a polygon featurelayer in arcmap. The coordinate system of the featurelayer's featureclass is web mercator. The dataframe of the map uses an equal-area projected coordinate system. I've ...
3
votes
1answer
147 views

Calculate area within Python script in Arcmap

I am trying to calculate the area of a polygon within my Python script. I create a new polygon from merging two together, and I'd like to add the area of the resulting polygon to a field in the output ...
3
votes
2answers
4k views

Cannot get areas for polygon layers in ArcMap 10

For an ArcGIS project opened in ArcMap 10, we have added polygon layers representing different geographical features. After all the polygons were created, we opened the attribute tables for those ...
3
votes
1answer
43 views

How to exclude some parts of raster classes based on size of area?

I have a raster file with 5 classifications. I've calculated area for all of the classes via Zonal geometry, Arcmap 10. Now, I want to exclude patches which are smaller than 100 sq. km. and ...
3
votes
2answers
368 views

Shapefiles for U.S. Area Codes (NANP)

I have a bunch of telephone numbers (just the area codes), and I'd like to geolocate them and make a map, but I haven't had too much luck googling for shapefiles for Area Codes. Surely these are in ...
3
votes
1answer
73 views

PostGIS: worldwide area calculation

I have a table with polygons in EPSG 4326. These polygons are distributed all over the world, for example: POLYGON((10.7324632658845 59.9253049236956,10.7331908150806 ...
3
votes
1answer
192 views

How to calculate buffer surface that falls within another layer's borders

The image below shows what two selected layers. One contains a series of borders, and the other a series of buffers. I am trying to obtain the buffered surface within each of the borders in the ...
3
votes
1answer
921 views

How to display Radius after Resizing a Feature in Openlayers

@captDragon has helped me with calling the coordinates of a polygon after I have resized or dragged the feature. Ineed the radius now, i have done a radius alert on my other map,Which works correctly, ...
3
votes
3answers
565 views

Overlay polygons on raster colour map to extract area values for each colour within each polygon

Does anyone know an answer to the following: I have a set of polygons created as the MCP of animal movements. I also have a habitat colour map raster, with 13 different habitat types as different ...
3
votes
1answer
69 views

Compute global DEM topographic indices and compromise projections

I would like to compute global topographic indices based on SRTM elevation data. These indices are computed along river/channel networks extracted from a DEM and are a functions of both local slope ...
3
votes
2answers
102 views

Determine density or cover from a raster layer that has been clipped by a polygon

I am a very new GIS user using ArcMap 10. I have a raster layer of tree canopy that I have clipped so that the layer is in a polygon that is a circle with a 2km radius. I have done this for four ...
3
votes
0answers
157 views

Polygonize vegetation area from Landsat7 RGB using QGis / Bilko?

I'm trying to extract the areas with vegetation as a polygon-shape from a landsat7-image, 30m resolution, RGB, using QGis 1.8 or Bilko or (hopefully in the near future) ArcGIS. The area is in Northern ...
2
votes
2answers
206 views

How to single out country data from a world data map?

Say that i have two "data sources"; A global map of average wind speeds, A shape file (or similar information) of all country borders in the world. How do i combine these two sources and extract ...
2
votes
1answer
719 views

Convert area from square meters to degrees globally

I have a global raster file with 3600x1800 pixels. I don't have any information about the projection used. The raster contains one band with a quantity with the unit "per square meter". How can I ...
2
votes
1answer
89 views

How to measure the area of polygons within a fishnet

I am currently trying to measure the area of polygons within each square of a fishnet. The aim of what I am doing is to work out the proportion of different agri-environment schemes by 1km squares. ...
2
votes
1answer
77 views

ArcGIS - merge polygons and get combined area

I cannot for the love of god figure out how to combine (join?) two overlapping polygons to determine the combined area they cover in ArcGIS 10. I am sure the Geoprocessing tools can do it, but just ...
2
votes
2answers
94 views

How to calculate area in sq km of an ellipsoid shapefile

In QGIS, I often find myself setting up projects with an ellipsoid coordinate system (e.g. WGS84) so that the resulting maps have lat lon scales, etc. Often in these projects I also need to do some ...
2
votes
2answers
140 views

How to calculate the area of rasters in QGIS?

I have been working with some rasters and would like to know how to calculate the area occupied by one element of a raster in Qgis.
2
votes
1answer
89 views

How to calculate volume from a raster file?

I have a raster file containing the depth of water. I need to find the volume of water using that raster file. I need to find the area of each cell and multiply with the depth. How can I do it in ...
2
votes
3answers
202 views

How to calculate exact area of a loaded jpg map in to arcmap?

I calculated the polygon area through arcmap.but i'm not sure that the area is not actual,may be scale is not right. so how to calculate the exact area of a jpg image?

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