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Questions tagged [gis-principle]

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

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606 votes
7 answers
576k views

Measuring accuracy of latitude and longitude

I have latitude and longitude as 19.0649070739746 and 73.1308670043945 respectively. In this case, both coordinates are 13 decimal places long, but sometimes I also get coordinates that are 6 decimal ...
Saurabh's user avatar
  • 6,187
262 votes
11 answers
154k views

Difference between projection and datum

What is the difference between a projection and a datum?
fmark's user avatar
  • 9,381
143 votes
14 answers
77k views

What are Definition, Algorithms and Practical Solutions for Concave Hull?

Convex Hull A convex hull of a shape is defined as: In mathematics, the convex hull or convex envelope for a set of points X in a real vector space V is the minimal convex set containing X (Wikipedia)...
Adam Matan's user avatar
  • 6,828
64 votes
7 answers
10k views

What strategies, criteria, or rules to use for selecting coordinate systems?

What strategies, criteria, or rules do you use for selecting coordinate systems for (a) storing, (b) analyzing, and (c) displaying GIS data? (I humbly offer my reply to a related question about ...
whuber's user avatar
  • 69.9k
34 votes
1 answer
9k views

How to create an accurate Tissot Indicatrix?

A Tissot Indicatrix is useful method for communicating at a glance the kinds of distortion a given projection is prone to (in the figure below, each of the red circles occupies the same area). I've ...
matt wilkie's user avatar
  • 28.2k
85 votes
2 answers
493k views

Calculating longitude length in miles

Suppose I have the geographic coordinates of "Saratoga, California, USA" as Latitude: 37°15.8298′ N Longitude: 122° 1.3806′ W I know from here that in the case of latitude 1° ≈ 69 ...
ABC's user avatar
  • 961
45 votes
4 answers
64k views

Deciding what interpolation method to use for resampling raster data?

Are there hard and fast rules about which interpolation methods are suited to each kind of raster data?
ninesided's user avatar
  • 605
21 votes
2 answers
23k views

Understanding terms in Length of Degree formula?

Online calculators such as http://www.csgnetwork.com/degreelenllavcalc.html (view page source) use the below formulas to get meters per degree. I understand in general how distance per degree varies ...
Brent's user avatar
  • 313
31 votes
8 answers
13k views

Calculating intersection of two Circles

I'm trying to figure out how to mathematically derive the common points of two intersecting circles on earth's surface given a center Lat/Lon and a radius for each point. For example, given: Lat/Lon ...
Will's user avatar
  • 311
34 votes
4 answers
8k views

How to represent trend over time?

I have a time series of rasters where the cells of each raster represents some value at a certain time. I would like to generate a map that highlights the magnitude of positive or negative trends in ...
user2856's user avatar
  • 66.7k
21 votes
5 answers
25k views

What is projection of WGS84 in GIS software? [duplicate]

WGS84 is a reference ellipsoid that determines coordinates of every point on Earth using latitude, longitude and height about the its surface. This ellipsoid is often used as a base to make maps. ...
multigoodverse's user avatar
27 votes
1 answer
7k views

Calculating Flow Direction and Delineating Basins from Projected vs. Unprojected DEM Data [closed]

This is somewhat of a theoretical question stemming from some discussions with colleagues on the topic of implications with delineating basins with projected (e.g., Albers Equal Area) vs. unprojected (...
MapBlast's user avatar
  • 1,127
20 votes
3 answers
28k views

How do you compute the earth's radius at a given geodetic latitude?

(I see there is an equation on wikipedia that does exactly what I am asking but there is no references. I have no way of confirming the validity of this equation!) I already understand the difference ...
Trevor Boyd Smith's user avatar
44 votes
7 answers
21k views

Comparing two spatial point patterns?

If I have two point pattern distributions within the same geographic region, how would I go about visually and quantitatively comparing those two distributions? Also assume I have many points within ...
Andy W's user avatar
  • 4,234
16 votes
3 answers
5k views

Removing elevation trend over sloped surfaces?

Is there a known methodology for removing the trend from a DEM surface? For example, if I had a DEM of a river, that river would flow from high elevation (upstream) to lower elevations (downstream). ...
Radar's user avatar
  • 10.7k
38 votes
3 answers
18k views

What is approximate error of Pythagorean Theorem vs. Haversine Formula in measuring distances on sphere at various scales?

Many people when first trying to calculate distances between two longitude / latitude pairs ask if Pythagorean theorem works as an appropriate distance function. Most often people answer "no, the ...
Amos Budde's user avatar
16 votes
2 answers
9k views

Projecting EPSG:4326 data in 2D map?

I like to think I'm relatively well-versed in datums, projections, and coordinate reference systems. OpenStreetMap data is stored in WGS84 Lat/Lon (EPSG:4326). This CRS is geographic and therefore ...
tomfumb's user avatar
  • 3,688
41 votes
3 answers
79k views

Differences between triangulation and trilateration

Looking around I noticed that many people interchange the terms (triangulation and trilateration) for the same sense. What is the correct sense of Triangulation and what are the differences from ...
adev's user avatar
  • 529
32 votes
4 answers
25k views

Difference between node and vertex in GIS?

In graph theory the term of node and vertex are equivalent, but sometimes in the GIS world the nodes and vertex are mentioned in the same context. I was wondering what the difference is? Node: can ...
user12282's user avatar
  • 503
25 votes
6 answers
10k views

Algorithms for matching segments [closed]

What are the best algorithms to match segments? I'm trying to match corresponding segments from two map sources, one less accurate but with segment names, and one more accurate without segment names. ...
Adam Matan's user avatar
  • 6,828
25 votes
7 answers
4k views

Spatial reference for dummies?

Can anyone recommend a good reference or tutorial that explains the principles of spatial references, projections, and coordinate systems? We are a small team of programmers working with GIS ...
17 votes
3 answers
15k views

What does M-value represent?

I am trying to wrap my ahead around the meaning of the M-value. I read ESRI's dictionary definition here: https://support.esri.com/en/other-resources/gis-dictionary/term/m-value and was not able to ...
NULL.Dude's user avatar
  • 2,141
14 votes
5 answers
3k views

Calculating visible sky percentage?

Can anyone explain, as simply as possible, how to calculate the visible sky percentage? I have done a satellite survey and have worked out the height of buildings and the distance between them. ...
user avatar
4 votes
1 answer
382 views

How can geographic coordinate system such as WGS84 draw on screen?

A Geographic Coordinate System represents coordinates on a sphere or ellipsoid. To draw these coordinates on the screen does GIS software apply a default projection method, or does it just consider ...
antonyzhang's user avatar
34 votes
2 answers
23k views

What are the Differences Between TMS, XYZ & WMTS?

There are currently three different ways of Serving out Tiles: TMS WMTS XYZ I do know that XYZ is now the most popular one, and is used by services like Google, Mapbox, OSM and many others. What is ...
Devdatta Tengshe's user avatar
18 votes
4 answers
2k views

Using GIS tools for non-geographical problems

Many other fields have visual/"spatial" problems that can potentially be solved with GIS tools. I know I've used GIS tools for the odd non-geographical task and was wondering how much this happens and ...
16 votes
4 answers
20k views

Meaning of "Elevation above surface of ellipsoid"?

I'm using implementation of Vincenty's algorithms, and I have their class called "position" that requires lat, lon, and elevation defined by: "Elevation above the surface of the ellipsoid". What is ...
michael's user avatar
  • 1,809
10 votes
3 answers
16k views

Calculating bounding box size?

What formula could I use to calculate the size in Kilometres of a bounding box based on a given South-West latitude/longitude and a North-East latitude/longitude points? The bounding box format is ...
Chris's user avatar
  • 209
10 votes
1 answer
15k views

Queen vs Rook neighborhood

Queen and rook neighborhoods are two common ways to calculate statistics for a focal cell. They are also known an Moore and (von) Neumann neighborhoods. I think a "bishop version", i.e. from lower ...
Jens's user avatar
  • 1,188
9 votes
2 answers
11k views

Understanding GeoTIFF tags?

I'm trying to understand GeoTIFF info, but not getting far. For instance, using listgeo with one file I can get this output: Geotiff_Information: Version: 1 Key_Revision: 1.0 Tagged_Information:...
Kyudos's user avatar
  • 243
4 votes
1 answer
95 views

Where do 'line and point' holes come from?

It's quite common to find these topology errors when dissolving data: Census tracts dissolved by neighborhood using ST_Union (PostGIS) It's quite simple to understand the concept of a polygon donut ...
Beatriz Viseu's user avatar
2 votes
1 answer
8k views

Measuring distances when crossing UTM zones?

I like to calculate distance between two UTM coordinates which are in different zones. How I can do this? I found that we can map these coordinates in to a new projection for solving the problem. ...
Chaitanya's user avatar
47 votes
6 answers
56k views

Using geohash for proximity searches

I'm looking to optimize point proximity geo searches time. My input is lat,lng point and I'm searching on a precomputed set of locations to n nearest points. I don't care how much time/space the ...
Maxim Veksler's user avatar
31 votes
2 answers
5k views

Benefits of Multipart Features?

Multipart points, lines, and polygons are implemented in nearly every GIS, but what benefits, if any, do they provide? In a relational database attributes shared by different features can be stored ...
geographika's user avatar
  • 14.3k
25 votes
2 answers
1k views

What is this GIS Principle Called?

I've forgotten the name of a particular GIS principle and need a refresher... Scenario: Let's assume there is some polygon which represents some administrative area. Let's say it's a square, for the ...
Fezter's user avatar
  • 21.9k
24 votes
4 answers
2k views

Recommend topics to be included in a Computer Science for Geospatial Technologies course

I will be instructing a course at the local university titled Computer Science for Geospatial Technologies. This is an introductory course meant to introduce computer science concepts to geospatial ...
15 votes
1 answer
485 views

Matching segments of different lengths

I'm trying to match small segments with a larger segment they are the most probably related to: relatively close, similar bearing, and facing each other. Here is a typical example of the data I have: ...
Jukurrpa's user avatar
  • 331
15 votes
2 answers
13k views

Difference between datum and ellipsoid for geodetic coordinates?

Looking at the Wikipedia page for the North American Datum, it seems to imply that the datum is completely defined by the ellipsoid, which defines the major/semi-major axes, flattening, etc. For ...
Nicolas Holthaus's user avatar
14 votes
2 answers
1k 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 ...
zabop's user avatar
  • 2,214
14 votes
2 answers
3k views

What is smallest contour interval possible when derived from DTM?

I am converting a 2m DTM to contours, and would like to have the smallest interval possible. Is there a theoretical limit, or even a rule of thumb for defining contour intervals based on the ...
Matthew Snape's user avatar
12 votes
1 answer
1k views

Is there cartographic convention to ordering legend items (high-low or low-high)?

When you symbolise a layer in ArcMap, the symbols are ordered from low value to high value by default. Is that necessarily "correct"? I've never officially studied cartography - is there a convention ...
Stephen Lead's user avatar
  • 21.1k
12 votes
1 answer
7k views

Raster vs Grid vs Lattice terminology?

I have a question about GIS terminology - what is the difference between grid and raster and grid and lattice - in theory?? I thought, that raster is regular network and value is in the cell and the ...
VilemRousi's user avatar
11 votes
2 answers
3k views

Fistance when viewing scaled-topographic map

A close friend from my company asked me a question about a topographic scaled map (1:25.000): What was the altitude to a location in km (as you are viewing Earth from space) when he was viewing the ...
urcm's user avatar
  • 22.5k
10 votes
4 answers
766 views

Is there a standardized or "most used" dummy Z value?

Creating and importing both 2D and 3D data, I have many times encountered the situation where I have no Z value for a coordinate set, that the value of a Z coordinate seems out of range (like -99, -...
Chau's user avatar
  • 4,480
8 votes
3 answers
3k views

Is the Fourth Dimension used in GIS?

Whilst working with OGR, I came across the following field used define a geographic dataset: COORD_DIMENSION The spatial dimension (2, 3 or 4 dimensional) of the column. Whilst I've used 2D and 3D ...
geographika's user avatar
  • 14.3k
8 votes
4 answers
3k views

How does a Spatial Reference System like WGS84 have an elipsoid and a geoid?

I found out that WGS84 is an ellipsoid with a "geoid component", but I'm unsure exactly how that can be. This is from PostGIS in Action, Second Edition (pg. 64), but it seemed a bit vague. ...
Evan Carroll's user avatar
  • 7,091
7 votes
2 answers
2k views

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 ...
David Kan's user avatar
5 votes
1 answer
151 views

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 ...
Sideshow Bob's user avatar
  • 2,343
5 votes
1 answer
245 views

Least cost path: what does least cost value represent, based on slope raster layer

Context and question I have a raster layer with slopes in percent and used the least cost path plugin in QGIS to calculate the least cost path from start to end. I try now to understand what the ...
Babel's user avatar
  • 71.9k
2 votes
1 answer
416 views

Computing global DEM topographic indices and compromising 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 ...
Maayt's user avatar
  • 29