# Questions tagged [gis-principle]

Use for questions which involve a GIS principle or theory and are software agnostic

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### Identify the algorithm (not code) for QGIS native tools

The question For someone with no knowledge in C++ (programming language used to write QGIS code), is there a way to identify based on which algorithms the different QGIS native tools are written? ...
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### Are contiguity matrix and adjacency matrix the same?

I'm am sorry for that beginner question. To start the calculation of a spatial auto-correlation value, the book I'm reading writes that we will starting to construct a contiguity matrix. Looking at ...
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### How is datum used in assigning lat and long to point on Earth's surface?

It's accepted wisdom that geographic coordinate systems require a datum in order to assign lat/long to a point on Earth's surface. However, every point in 3d space already has 3d polar coordinates ...
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### Spatial indexes with RTree for polygons

I am trying to understand about how R-tree works for polygon data. In other words, polygon is a list of vertices (points), however, I am wondering how spatial index can recognize the order of points ...
33 views

### Is there a relationship between driving distance and straight line distance?

For fun, I created an FME workflow that emails me the 10 cheapest fuel princes within 10km of my house. I calculate both the driving distance along roads and the straight line distance. Knowing that ...
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### MultiSurface Geometry type in QGIS: difference to MultiPolyon

Adding a new temporary scratch layer in QGIS, selecting the Geometry type, there are different options, between else MultiSurface. In the documentation, I could not find any information about this ...
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### Determining scale for spatial aggregation based on precision of lat lon coordinates

I need to retrieve terrain attributes from DEM such as aspect and slope representative for a point coordinate. I have realized that the coordinates that I have might be too coarse to make a meaningful ...
18 views

### Finding(drawing) interior, boundary and closure of given 2D visual representation of topological space

I'm currently studying for my exam in Geospatial Databases. We're given a visual topological space (picture below) and we need to draw the interior, boundary and closure of this space. I understand ...
164 views

### How to interpret Tobler’s First Law of Geography?

Waldo Tobler's First Law of Geography is an axiom in GIS which states that "everything is related to everything else, but near things are more related than distant things" (Tobler, 1969). ...
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### Confusion in longitude range, why do they start at 80? [closed]

In one of our notes in the university, there's a paragraph about latitude and longitude that tell us this: Geographic coordinates are the earth's latitude and longitude system, ranging from 90 ...
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### Naming convention for ways of defining direction

The following two conventions for defining direction are very common, but what are they called? 0 at north, clockwise 0 at positive x-axis, counterclockwise Background: I'm writing a function that ...
• 141
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### Higher resolution of country borders

I have been looking and looking for a long while, but I was unable to find any freely available datasets for country borders with resolution higher than 1:1000000. I found this strange as I thought ...
684 views

### Why accurate area of all countries cannot be obtained using a single EPSG code, even if it corresponds to an equal-area projection?

I would like to check if using different equal-area projections lead to near-identical area estimation for a list of countries. Importing modules: import numpy as np import geopandas as gpd import ...
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1 vote
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### Are there any mass-preserving raster projection algorithms?

Raster maps (e.g. as a GeoTIFF) can be reprojected using tools like gdalwarp. While these tools do a decent job to project images, they are not truly mass-preserving, i.e. the total sum of the values (...
1 vote
304 views

### Efficiently converting/storing WGS84 Lat/Long as Integers

I'm a GIS newbie and I'm trying to figure out how I might store WGS84 coordinates as an integer efficiently. I could simply multiply it (10e7 for example) but I feel like this must be a problem that's ...
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### Accuracy of raster cell size area to real world area

I read from somewhere here that a UTM raster's pixel count ratio is 1 pixel = 1 meter. I am working on a raster that says the island is 4.03 km² in area but a quick google search says the island is 4....
82 views

### Single numbers for accuracy and resolution for vector and raster data?

This question is meant as a modelling question, not having to do with particular data formats. It was edited to better separate spatial accuracy and resolution (precision). In order to make the ...
202 views

### Why does cubemap projection preserve lines of longitude but not latitude?

My understanding of cubemaps is as follows: To create a cubemap from a sphere, we project from the centre of the sphere outwards, onto the faces of a cube. Each face of the cube therefore represents ...
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### Is there a raster data format that can store dates instead of numeric values?

When storing spatiotemporal data, there is a need to store dates or time data. I know raster files can store numeric data (integer or float), but what about storing of dates data? I'm currently ...
310 views

### What is the principle of pooling in ArcGIS Server?

If I understand correctly, pooling an AGS Service means that when a client connects to a service, an instance is created and an ArcSOC process is executed. Setting minimum to 0 thus doesn't start the ...
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1 vote
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### Why are some maps shown with polar coordinates (azimuthal) and some Cartesian (cylindrical)

I am very new here, and in no way am I a cartography expert like all of you here. I am currently doing a math paper on map projections, and I have noticed that some projections (mainly azimuthal) are ...
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1 vote
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### Is topology stored explicitely in vector layers or is it derived from stored geometry information?

In vector layers, different types of information are stored like: Geoemtry Attribute data Meta data (like filename, projection etc.) Spatial indices However, what about topology? Is it an explicit ...
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### What's the theory (principle) behind shapely.ops.polygonize?

I am learning Python package Shapely, but quite confused about the principle behind the polygonize function --- shapely.ops.polygonize. How does this function work to polygonize a set of lines (...
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### Understanding the concept of m value

As far as I understand, the m value "is a measured distance along a route" (similar answers here), thus similar to the mileage of a car driving over a highway. The m value defines the point ...
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### Figure out location of city direction sign in image - friday fun project

So I came across this image and wondered where it could be located given the directions and distances. But I started to realise that the sign is all wrong. I'm guessing the Greek alphabet one is ...
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### Why does the same UTM northing give different values when converted to latitude?

I'm converting two coordinates from UTM to latitude/longitude. The coordinates are: Easting: 657787 Northing: 7795664 and Easting: 658144 Northing: 7795664 Both from zone 23S (EPSG:31998) or 23K (...
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1 vote
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### Storing information for each segment of spatial line

I have some spatial lines from a shapefile representing roads. I'd like to attribute to each segment an information, for example the state of the segment or more generally a scalar that gives an ...
• 7,523
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### Concept behind longitudes

Why longitudes are not parallel and why do they meet at the poles? What problem would have happened if like latitudes they were parallel circles running from west to east? I would request the ...
566 views

### Understanding map projection coordinate systems

I have been playing around with a variety of map projections for the purposes of transforming between various representations. The thing that I still don't quite understand is how to interpret the (x,...
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1 vote
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### Checking if network has connection errors

I have given a network of a city in dwg. I made a dxf that only contains the network itself, and created a layer for every type of pipe (like 25KPE, 80KPE etc...). How do I check if the network is ...
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### Determining if two locations are duplicate

I'm working on an app that will display drugstores in a map. I have a table that has all drugstores, each with its Lat/Lon. We used several different sources to add the data to the table: the ...
1 vote
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If I take a Natural Earth shapefile and delete all countries except one - This will affect that original shapefile. Can I take that shapefile duplicate it and use that shapefile to delete all ...
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### Seeking documentation about non-iterative UTM to Lat-Long projection formulas

In Hager, John W., James F. Behensky, and Brad W. Drew. "The universal grids: Universal transverse mercator (utm) and universal polar stereographic (ups)." DMA technical manual 8358 (1989). I ...
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1 vote
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### Deciding when to use projected coordinate system (PCS) or geographic coordinate system (GCS)? [duplicate]

While I understand the technical differences between them both, I'd like an expert's breakdown of criteria to look out for that would determine when I should use a PCS or GCS. I understand that a PCS ...
683 views

### Average Cross Slope Calculation

My county ( Pima, Arizona ) uses a number called “average cross slope” which they define as ACS = (I * L)/A where I = contour interval L = length of contour lines in the area A = area ...
66 views

### Assessing if two variables (measured at point locations) covary in space?

I have a number of (point) locations at which two variables are measured. The two variables proves correlated (when using Spearman's r; the direction of the correlation is irrelevant here). How can ...
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### Splitting linestrings based on their curvature

I'm looking for a method to split linestrings based on their curvature, more precisely if part of them could be considered a street corner. For example in the screenshot below, I'd need to split the ...
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1 vote
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### Creating contour file from DEM?

Using a 3-meter DEM, can I create an accurate 5 feet interval contour file? Or is the minimum interval contour I can get is 10 feet (roughly 3 meters)?
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1 vote
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### Applying a two step floating catchment with boundary limitations

The literature states that gravity models and the two step floating catchment area (which in itself is a version of the gravity model) method for analysing the accessibility of healthcare services by ...
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### What does first requirement for LISA statistic mean?

According to Anselin, a LISA statistic have to satisfy two requirements: The LISA for each observation gives an indicator of the extent of significant spatial clustering of similar values around that ...
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I am tasked with building a geocoder (and in the long term reverse geocoder) from a local data I own. Since I am new to this and literature is relatively scarce, I would like to know what are some ...
1 vote
115 views

### Finding glider reachable area / catchment area with custom slope?

I'd like to produce maps for gliders in mountains. I'm looking to display the reachable area by a glider taking off from a given point (obviously with the corresponding DEM). For a first approach I'...
61 views

### Create new properties when updating a layer

I'm trying to update a property that doesn't exist in my current database, and I want that if it doesn't exist - then create it with my value and only for this specific layer. For example - I have a ...
45 views

### The manual Dempster–Shafer theory in GIS? [closed]

I am looking for the manual Dempster–Shafer theory. Can you help me?
1 vote
5k views

### Turning degrees into square kilometres (km2)?

After running raster layer unique value report I have got pixel number and area in degree. How can I turn it in square kilometres (km2)?
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### Difference between Geodetic Distance and Great Circle Distance?

According to Wikipedia: The great-circle distance or orthodromic distance is the shortest distance between two points on the surface of a sphere, measured along the surface of the sphere (as ...
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### Calculating haversine versus checking latitude and longitude digits after decimal point?

If I want to tell whether 2 points are close to each other, say at proximity of 50 meters, is it OK to compare both numbers up to a certain decimal point, for example the two points: 36.391155, -56....
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### Intersection and area calculations in geographic vs. projected coordinates

I think one should do intersection calculations when layers are in the same coordinate system, and one should do area calculations in the appropriate projected coordinate system. The order matters, ...
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### Understanding Inverse Distance Weighting Bulls Eye Effect?

Is the Bulls Eye Effect generally caused by a sample point with a high value being too far from other sample points? Or rather a sample point with a higher value than others close to it? What ...
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