# Tagged Questions

A mathematical procedure used to solve problems with a series of steps. Algorithms are usually encoded as a sequence of computer commands.

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### J2000 to WGS84 in QGIS?

How to transform Earth-Centered Inertial J2000 CRS to WGS84 CRS in QGIS? I searched J2000 in spatialreference.org, however nothing found. I don't know the proj4 string of J2000. Any advice would be ...
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### Is there an algorithm for transforming from CGCS2000 to WGS84?

Is there an algorithm that supports transforming from CGCS2000 coordinate to WGS84 coordinate? CGCS2000 is China Geodetic Coordinate System 2000. Any advice would be thankful!
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### How to assess quality of the TSP algorithm?

I'm trying to develop my own algorithm for solving Travelling Salesman Problem (TSP) I tested it (in its current state) on the "att48" instance of the TSPLIB and got following results: As we can ...
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### Aggregating river basins

I am not a hydrologist but am providing GIS infrastructure to our hydrology department which actually doesn't use GIS very much yet. Therefore I am asking for some hydrology related insights. Btw: We ...
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### Which algorithm does gdal_polygonize use?

I would like to know which algorithm (i.e. Sobel operator ) is used to extract the polygons in gdal_polygonizer. I searched here and here but could not find it. Any help would be appreciated.
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### Algorithm to find the boundary grid points [duplicate]

Let us consider a set S consisting of a collection of grid locations (i,j),i,j=1,\ldots,n, in 2-dimensional gridded plane. Here the point (i,j) corresponds to a rectangular grid in the X-Y plane. ...
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### using MCA in GIS for suitability analysis

I have an essay about using MCD methods to solve suitability analysis. I must write a scenario about fuzzy membership, WLC and AHP. I feel that I’m confused about when and where to use each method. ...
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### Free implementation of Polynomial Approximation with Exponential Kernel (PAEK) algorithm

I need a Free (i.e. Open Source) implementation of the Polynomial Approximation with Exponential Kernel (PAEK) algorithm, preferably in C, C++, Python, Julia or R. The algorithm is one of the methods ...
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### How to find the the boundary co-ordinates from a given set of coordiantes? [duplicate]

How to find the the boundary co-ordinates from a given set of co-ordiantes? I understand that this question asked previously and answered here How to find the boundary co-ordinates from a given set ...
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### Fast raster combine using rasterio

I'm trying to duplicate the functionality of ArcGIS's Combine function within rasterio. Given a list of (presumably categorical) rasters read with rasterio, I want to output a combined raster and a ...
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### Tools to generate all the possible geometries meeting specific constraints

I have an area = 2d polygon (generally a 4-side shape). I would like to generate all the possible rectangles matching the following constraints: that fall within the surface of the area with a ...
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### How do the equations/algorithms behind buffering tools actually work?

I've been wondering how the equations/algorithms behind buffering tools actually work?
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### Buffer Zone Algorithm

I use google maps, php, mysql, and I want to create a buffer zone in line among some points. Could someone tell me where can i find the algorithm (php library or the math in order to implement it) to ...
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### How to calculate grid convergence (True North to Grid North)?

I would like to know how to calculate the grid convergence angle between True North and Grid North for UTM maps. I am using ArcGIS 10.0 (ArcView licence) and have used the Calculate Grid ...
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### Buffering/extrusion of a dem surface

I have a surface representing a DEM and i would like to add it some thickness. If i simply duplicate the surface and translate it down by my thickness, i get non uniform thickness, ie thinner where ...
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### Find a path between an origin and destination given scattered points

We have a postgis database containing a lot of ordered (by date/time) nautical point (lat/long). Each of these points are affected to a leg between an origin and a destination (both origin and ...
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### PRN (p-code) to Geocode Conversion

I think I have a unique situation. Client has a file containing the old p-code / PRN but can't relate this to a specific address and desires to move to a geocode orientation. Does anyone know if a ...
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### Locating Moon in Sky from Earth

I am working on project which need to find moon location from a specified lat, long on earth. So basically info I am having is Lat,Long and Timezone. Format of the moon location Info? So first in ...
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### Interpolate data from multiple gpx tracks

I have a number of GPX tracks of the same road. But due to GPS error they are not accurate enough (20-30m off). I'm wondering if I could interpolate those multiple gpx tracks into single one. If an ...
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### How to get Intersection point from two lines (coordinates)

I have tried for ages to find the algorithm for getting the coordinate of two intersecting lines from a given coordinate (latlong). As the picture below suggest. I have two coordinates (A and B). ...
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### Calculating the fastest routes and some points on the same time

I have two drivers and each driver has a set of about 15 locations where they need to drive to on a single day. Calculating the shortest route for both drivers is not the problem (using a matrix ...
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### How do I map points on a 2D map to a 3D illustration of the same area? [duplicate]

I'm looking for advice on how to build an algorithm that maps any point on a map to a point on another map, interpolating where needed. Let me better clarify with an example of what I concretely ...
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### D8 algorithm to get the flow direction in ArcGIS

I am using ArcGIS 10 for the get the flow direction from DEM. I wonder about the algorithm they have used while calculating flow direction. It is mentioned there that it is using D8 model. D8 model ...
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### What is an efficient Algorithm to draw a straight line from points?

I have some cross section survey data, which is like this: X Y Z pt_dt 2970969.635 359725.0088 83.4242 1-x 2970968.278 359722.2182 83.2591 1-x 2970941.771 359670.127 83.0655 1-x 2970961.369 ...
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### Is it possible to create a standalone model in ArcMap Modelbuilder?

Is it possible to create an algorithm in ArcMap Modelbuilder that could then become a fully standalone applet? I would like to be able to provide this processing algorithm even to users who do not own ...
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### Step by step contour creation algorithm

I do seismic processing, and recently have been moving into doing contours. I have a question on the exact way this is done. I have watered down my problem so that I can understand from 'first ...
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### Advantages of R-trees in comparison to geohashes

Geohashes are being widely used in products like: Lucene, mongodb, etc and have become one of the most important technology of present day. Have Geohashes replaced the good old R-trees or do R-trees ...
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### Algorithm to find geohashes which satisfy query condition

I have an application where I need to query geohashes by specifying the bounding rectangle. Since geohashes map 2D to 1D is there a method by which I may find the prefixes of all geohashes which ...
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### Calculate distance in point collection algorithm

I've got collection of points which are sorted by x,y and distance between n and n+1 (array of objects [{x:0,y:0},{x:1,y:1}....] I would like to calculate distance between all points (point 1 distance ...
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### Counting number of points in 1 km grid covering surface of Earth?

I want to cover surface of earth with equal sized shapes and count number of points within each cell. shape of cell doesn't matters (triangle, hexagon ...), neither area, but equality of them is ...
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### How can I find a Douglas-Peucker algorithm for simplification? [duplicate]

I want to understand simplification methods in GIS. I searched the Internet and found that the best simplification method is the Douglas-Peucker algorithm. I could not find any information about the ...
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### Topographic Index - Why is upslope contributing area divided by downslope contour width and not upslope contour width?

In the Topographic Index calculation, a is the upslope contributing area per unit contour length. Is the "unit contour length" calculated from examining the contour widths of the downslope cells that ...
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### Determining if a Geodetic location is inside a polygon made up of Geodetic vertices

Suppose we have a polygon with Geotic vertices (Points A through E). Is there an algorithm that I can use that could determine If F is inside of it? In addition, what is the most efficient way to ...
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### Test for polygon orientation

I'm want to merge multiple geometries with Geotools (starting off with one geometry) but the result is a polygon that has a clockwise orientation. This gives a invalid geometry. My code for the merge: ...
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### How to select all files in a directory for merging in Processing?

I'm writing a generic script which involves writing shapefiles to a directory which are then merged together. After writing the files out to the Output folder, I'm trying to get the ...
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### Competing approaches to line & polygon simplification

There seems to be a few theoretical options around at the moment for geometric simplification. Douglas–Peucker seems the most established, followed by Visvalingam's approach (used by d3.js). Now ...
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I wrote the following code to start using QGIS APIs. in a very short script seen below, the "Error: Algorithm not found" is thrown and "print processing.alglist()" is showing none. Any idea why this ...
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### How to detect turns of a route?

I'm doing a Python simulation of a vehicle, driving along a specified path. I generate dummy GPS readings, that fetch the current position of the vehicle, with some randomized error added. After a ...
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### How to add nodes into graph

I'm new to qgis and in here I want to find a path between two selected points on the map(Roads-vector layer). The points are selected by the user, using mouse clicks. So here I used the astar ...
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### 'QgsGraph' object does not support indexing

Traceback (most recent call last): File "/home/kasun/.qgis2/python/plugins/floogin/floogin.py", line 246, in findRoutes path=self.astar.astarSearch(graph, start, end) File ...
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### Multilateration Code

I am currently writing some code to do multilateration using TDOA and currently struggling and wondering if anyone has any reference code that can be used as a test case with some sample data that I ...
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### Convert ECF to Suface ECF

I'm using the 3D Earth WorldWind Model and I have a path that I want my object to travel. I set the the destination and interpolate to the end point from my current location. The problem is using ECF ...
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### Trouble Using Selection Expression in QGIS

I am trying to use the selection expression box to highlight counties by state FIPS. When I use the OR conditional all counties across the US get highlighted. So for example, STATEFIPS = 1 will ...
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### What steps does SAGA GIS take when calculating Topographic Wetness, specifically when handling depression filled (by ArcMap) DEM

I am a grad student involved with hydrological modeling. In the model I work on, we are calculating the Topographic Index (TI; without transmissivity - or set to 1) using Quinn's MFD algorithm. We do ...
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### Looking for a simple hatching algorithm

If I have an arbitrary polygon, and wish to hatch it with lines at a given angle and spacing, is there a simple low-level algorithm which will return the set of hatch lines inside the polygon?
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### What's wrong with my IDW interpolation function (Python)?

Original Question Can anyone see what's wrong with my custom IDW interpolation below? I know there are existing libraries that can do this for me, but for various reasons including educational and ...
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### How to Calculate Geolocation Intersection of Two Vectors?

I'm unfamiliar with much GIS terminology but am tasked with solving a specific geolocation problem. For what it is worth, this is for an iOS project, so the data I can collect for analysis is that ...
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### Calculating Topographic Wetness Index (choosing from different algorithms)

Topographic Wetness Index can be expressed as Ln(a/tanB) based on the idea of Beven and Kirkby (1979) where a is the spesific catchment area (a=A/L, catchment area (A)divided by contour ...