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84

Geographic coordinate systems (lat/long) are based on a spheroidal (either truly spherical or ellipsoidal) surface that approximates the surface of the earth. A datum typically defines the surface (ex radius for a sphere, major axis and minor axis or inverse flattening for an ellipsoid) and the position of the surface relative to the center of the earth. ...


57

You will obviously get better answers from textbooks, but here is an simple explanation: Map Projection: It is a method for representing a spherical or curved surface on a flat plane. Datum: It is the reference or origin based on which measurements are made.


39

After struggling with this question ten years ago, and finding many confusing things written about the topic, I published a brief article in Directions Magazine that presented an answer as simply, plainly, and accurately as I could make it. The following is excerpted from that article. Reprojecting geographic features Two things must happen when you draw a ...


30

As scw points out, you want an implementation of α-shapes. Alpha shapes can be considered a generalisation of the convex hull. They were first described in 1981 in: Edelsbrunner, H.; Kirkpatrick, D.; Seidel, R.; , "On the shape of a set of points in the plane," Information Theory, IEEE Transactions on , vol.29, no.4, pp. 551- 559, Jul 1983 ...


23

Here is what you are looking for. You can download and test the program: (in java, under GPL license) The paper presenting the algorithm is there: Duckham, M., Kulik, L., Worboys, M.F., Galton, A. (2008) Efficient generation of simple polygons for characterizing the shape of a set of points in the plane. Pattern Recognition v41, 3224-3236


22

wwnick's answer is correct, but it is a bit misleading in the sense that it emphasizes ellipsoid parameters and IMO understates the importance of 'the position of the surface relative to the center of the earth' - the NAD 1927 example needs to mention that the geodetic "center" of NAD27 is a base station at Meades Ranch in Kansas. One could have (and often ...


17

This seems to be a specific application of alpha shapes, which are from my reading a more general form of this problem. R has the alphahull module, which has excellent documentation on computing alpha shapes. Also check this detailed background on alpha shapes. If you only want to compute convex/concave hulls, check out lasboundary, part of lastools, it ...


11

I created a highly-efficient tool, called [lasboundary][1,2], that computes a concave hull for LIDAR in LAS/LAZ/SHP/ASCII format and stores the result as a vector boundary polygon in ESRI Shapefile format or a geo-referenced KML file. Here is an example run: C:\lastools\bin>lasboundary -i SerpentMound.las -o SerpentMound_boundary.shp reading 3265110 ...


10

The Hausdorff distance may be used: matching segments could be 'close' segments according to this distance. It is quite simple to compute on segments. A free java implementation is available in JTS - see here. You may also have a look at the JCS Conflation Suite.


10

It seems that you are looking for examples of Agent-based Modeling (ABM). There are many GIS models adopt the ABM mechanism. For example, urban planning used lots of cellular automata models that are essentially same as the flocking model. I have implemented a ABM for U.S. logistics industry using AnyLogic to detect the dynamic organizational structure for ...


8

I don't know what would be the "best," because that will depend on the particulars of your segments. A generally good approach is to hash the segments into crucial geometric information. This would include, at a minimum, location of the center (x,y), orientation (0 to 180 degrees), and length. With appropriate weights applied, and some finessing of the ...


8

There is an implementation of ST_ConcaveHull in PostGIS trunk. http://postgis.net/docs/ST_ConcaveHull.html /Nicklas


8

Geographic projections are a way of showing the curved surface of the Earth on a flat surface like a piece of paper... From the Manifold user documentation: Earth is not an exact ellipsoid. In fact, because the Earth is such a "lumpy" ellipsoid no single smooth ellipsoid will provide a perfect reference surface for the entire Earth. The practical ...


7

Other replies in this thread show that some specialized datums do depend on the earth's magnetic field. However, geodetic datums are determined ultimately by the earth's gravitational field, which establishes the "geoid" (an idealized "sea level," or contour shell of gravitational equipotential). The geoid is then approximated by an ellipsoid of revolution ...


6

The problem is that the names sometimes change depending on the software. Below you find the definitions from ESRI. RASTER = A spatial data model that defines space as an array of equally sized cells arranged in rows and columns, and composed of single or multiple bands. Each cell contains an attribute value and location coordinates. Unlike a vector ...


5

This won't compete with wwnicks answer and not rigorous, but the visualization I present to people, when asked, is the relationship between a string connected to a ball. Changing the projection is often like moving the 'loose' end of the string around, but still connected to the same point on the ball. Changing the datum is like changing the location of the ...


5

I think the four dimensions used in GIS are x, y, z (height), m (measure). This measure can be time or something else like the projected distance along a reference line, e.g. a pipeline.


5

About R implementation Alpha-Shapes, there's an article about "Converting Alpha-Shapes into SP Objects" It's based on alphahull, sp and spgrass6 http://casoilresource.lawr.ucdavis.edu/drupal/node/919


5

James Macgill, Stan Openshaw and I did some more work on cluster detection using flocking boids back in 1999 http://www.geocomputation.org/1999/069/gc_069.htm. This seems to have been followed up by Gianluigi Folino http://staff.icar.cnr.it/folino/papers/ppsn02.pdf. There is also Jameson Conley's work who was a student of James.


4

Here is an R function that implements the Alpha Hull model. The output is an sp polygon object. Please see the example in the header. It requires the sp, alphahull and maptools packages. ###################################################################################### # PROGRAM: ConvexHull # USE: CREATE CONVEX HULL USING THE Pateiro-Lopez ALPHAHULL ...


4

I've written code to handle sloppy line segment matching (and overlap them) in Boundary Generator. I wrote up the (fairly elementary) math behind it here: http://blog.shoutis.org/2008/10/inside-boundary-generator-computational.html. The code is open source & linked from that blog post. The code follows a really simple approach: A segment-segment test ...


4

Here comes an idea If you tear apart one of the linestrings to compare and test if the vertexpoints is within some distance from the other linestring to compare you can control the test in many ways. those examples work in PostGIS (who could guess :-) ) First, if we say that there is a match if all vertex points in a linestring in table_1 is 0.5 meters ...


4

I worked on a project with a similar requirement about 5 years ago. It involved combining coordinates from street centerlines (with relatively high coordinate precision) with Highway Performance Monitoring System (HPMS) traffic network links. At the time the FHWA didn't provide any tools to do this sort of thing. That may have changed, you might want to ...


4

Speaking about JTS, you can use Geoscript for manipulating JTS library. http://geoscriptblog.blogspot.com/2010/06/unwrapped-jts-with-python.html for an article about convex hull. GeoScript implementations are available in JavaScript, Python, Scala, and Groovy. The official website : http://geoscript.org


4

JTS (http://tsusiatsoftware.net/jts/main.html) provides a Convex Hull implementation. Martin Davies also mentioned having an Alpha Shape algorithm in the works so you might want to check the SVN repository to see if it is in yet if that's what you want.


4

Good accounts of algorithms in 2 and 3 dimensions appear in the classic text by Preparata & Shamos. Algorithms used in GIS are a specialty of Hanan Samet, who has published several books on the subject. Higher-dimensional searches are usually assisted or sped up by means of preliminary data mining, clustering, or dimension-reducing techniques. This is ...


4

The classic (paleogeographer) answer is to use a K-D tree to store the data in (see http://en.wikipedia.org/wiki/Kd-tree). These work by roughly halving the data in to two partitions in each dimension in turn as you move down the tree. The advantage of them is that as you find the nearest item you can also create a list of nearest items as you go for no ...


4

I've always referred to "Map Projections: A Working Manual", 1987, Snyder, John P. USGS Professional Paper: 1395 which is available as a PDF to download.


3

Think of projection as seeing your location on X/Y plane. Datum defines the reference point from where all measurements were made. Say you are located somewhere and need to tell your location to someone. You would say, i am X lat and Y long. This X and Y are deterministic because they are being referred from the Datum. The other person now knows that you are ...


3

In this case the forth dimension is measure as gissolved mentions, but more commonly the fourth dimension does refer to time. Historically, most GIS systems have been weak at integrating time, but the increase in dynamic modeling over the years has brought time into most current GIS systems. See for example this recent question. While the software has been ...



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