# Tag Info

94

Here's my rule of thumb table... Latitude coordinate precision by the actual cartographic scale they purport: Decimal Places Aprox. Distance Say What? 1 10 kilometers 6.2 miles 2 1 kilometer 0.62 miles 3 100 meters About 328 feet 4 10 meters About 33 feet 5 ...

82

It doesn't matter at what longitude you are. What matters is what latitude you are. Length of 1 degree of Longitude = cosine (latitude in decimal degrees) * length of degree (miles) at equator. Convert your latitude into decimal degrees ~ 37.26383 Convert your decimal degrees into radians ~ 0.65038 Take the cosine of the value in radians ~ 0.79585 1 degree ...

63

POINT #1. lets differentiate Precision from Accuracy As it is clear from the picture we can talk about Accuracy of a measurement (e.g. GPS measurement) if we already know the actual value (exact position). Then we can say how accurate a measurement is. On the other hand if you have some measurements and don't know the actual value you can just talk about ...

39

I think this XKCD is a perfect answer to this question :) https://xkcd.com/2170/

38

Using the Pythagorean formula on positions given in latitude and longitude makes as little sense as, say, computing the area of a circle using the formula for a square: although it produces a number, there is no reason to suppose it ought to work. Although at small scales any smooth surface looks like a plane, the accuracy of the Pythagorean formula depends ...

34

I'll try to explain it in different terms: Earth's equatorial circumference is about 40,000 kilometers (25,000 miles). A latitude/longitude value breaks that distance up into 360 degrees, starting at -180 and ending at 180. This means that one degree is 40,000 km (or 25,000 miles) divided by 360: 40,000 / 360 = 111 25,000 / 360 = 69 (So, one degree is ...

29

These two illustations are from the field of surveying but they should still apply. Triangulation As Martin has said, in triangulation, you work with angles as illustrated in the following figure. The positions of the points of interest are computed based on measured angles and two know points. From those angles, the distances are computed which are in ...

28

The principal radius of the WGS84 spheroid is a = 6378137 meters and its inverse flattening is f = 298.257223563, whence the squared eccentricity is e2 = (2 - 1/f)/f = 0.0066943799901413165. The meridional radius of curvature at latitude phi is M = a(1 - e2) / (1 - e2 sin(phi)^2)^(3/2) and the radius of curvature along the parallel is N = a / (1 - e2 sin(...

28

General Geographers among other scientists seek for geographical patterns hoping that this will help them to better understand the processes that have produced these patterns. As you shown, this process begins with the mapping of the locations at which the phenomena are located. Oftentimes, such maps as you have produced above are known as point pattern ...

24

Reprojection in GIS consists in changing the coordinates values of a dataset from one coordinate system to another coordinate system. Let's take a one dimensionnal case as an exampple. Imagine that you need to guide two friends who are trying to find your house. The first thing that you need to know is from which direction they will approach your street. If ...

23

WGS84 doesn't define a projection, so it's up to the GIS software to decide which projection to use for displaying the data on the screen (unless you manually pick a projection, of course). In the simplest case, a plate carée projection (i.e. equidistant cylindrical with standard parallel 0°) is used, which in essence just interprets the angular units of ...

21

It's not much harder on the sphere than on the plane, once you recognize that The points in question are the mutual intersections of three spheres: a sphere centered beneath location x1 (on the earth's surface) of a given radius, a sphere centered beneath location x2 (on the earth's surface) of a given radius, and the earth itself, which is a sphere ...

20

The elevation above the ellipsoid (ellipsoidal height) is the elevation above a mathematical model that approximates the shape of the earth. The current most common one is WGS84. These are the elevations that you'd get from a GPS. Orthometric heights are measured above the geoid or equipotential surface, that is, the surface of equal gravity. MSL is "mean ...

17

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 ...

17

I often refer to the ESRI GIS dictionary in these cases. Based on these definitions, nodes have topology whereas vertices do not. Vertex: [Euclidean geometry] One of a set of ordered x,y coordinate pairs that defines the shape of a line or polygon feature. Node: [ESRI software] In a geodatabase, the point representing the beginning or ...

17

The only reasons I can see why it would not be appropriate to use a vertical scale bar would be: The audience does not want it or does not understand it The audience associates the vertical scale with elevation For a simple way to make a vertical scale bar (tested in ArcGIS) - create a horizontal scale bar, convert it to graphics, and rotate +-90 degrees. ...

17

This is perfectly normal behaviour in a transverse Mercator projection. The fact that a specific northing does not match a specific latitude (except for the Equator itself) can be easily visualized. We are used to seeing global maps of the more familiar equatorial-aspect (or normal) Mercator projection, which depicts parallels and meridians as perfectly ...

16

I wrote an in-depth article on this on my blog here: http://www.sharpgis.net/post/2007/05/05/Spatial-references2c-coordinate-systems2c-projections2c-datums2c-ellipsoids-e28093-confusing It covers all these concepts in a hopefully easy to understand manner, and has been peer-reviewed by several. To sum it up: A datum is a definition of the size, ...

16

It may surprise you to find that most people likely do perceive your hillshaded DEM as containing depressions, however, this is not an uncommon experience. Here is the reason why. Ever since the earliest days of shaded relief topographic mapping, cartographers have been aware that the perception of elevated land versus low-lying land depends on the direction ...

15

Plotting the estimated slopes, as in the question, is a great thing to do. Rather than filtering by significance, though--or in conjunction with it--why not map out some measure of how well each regression fits the data? For this, the mean squared error of the regression is readily interpreted and meaningful. As an example, the R code below generates a ...

15

No, a datum and ellipsoid are not equivalent. For a loose definition, think of the ellipsoid as defining size and shape. The datum then fixes that ellipsoid to the earth. NAD83 (various realizations) and WGS (another set of realizations) use almost the same ellipsoid GRS80/WGS84, and were originally designed in the 1980s to be equivalent. Since then, NAD83 ...

15

One reason would be (I think it is mentioned in one of the comments) is that the vertical scale bar may not accurately represent the distance of the railway track. This would be due to the likelyhood of curves within the track itself. A possible alternative to a vertical scale bar is to label the aggregated distance at each specific point of the track, ...

12

A geographic projection will always be in degrees (or possibly radians) (the distinction between decimal and dms is one of formatting). A projected crs can be in any units the designer wants, often meters or some kind of feet but I have seen "German Legal Metres", chains and many other weird options.

12

The term resolution refers to the smallest details that can be distinguished. It is mainly used for raster data (resolution in time, in space or in spectral domain). For a vector map, two other concepts are more broadly used: the scale and the minimum mapping unit. The scale is related to the spatial precision of the boundaries of your entities. It comes ...

11

A projection is used to 'flatten' the ellipsoidal shape of the earth to rectangular coordinate system (i.e, to make a 'roundish' globe into a flat map). A datum is a specific, known location on or in the Earth that is used as a reference. All GIS coordinate systems use a datum as a point of reference (i.e., it's location on Earth). There are two types of &...

11

We should remember the earth is not a simple sphere, if it was, we need one datum "= One calculation system to find a point on earth", earth is more ellipsoid, but not exactly. Earth is an astronomic geoid without a regular shape, so we may have many ways to calculate coordination of a point in this irregular 3D object, with many opinions and concepts, each ...

11

Generally to calculate the area of a bbox in a projected coordinate system since it's a (big) rectangle you can use the area formula : area = (sw_longitude - ne_longitude) * (sw_latitude - ne_latitude) Depending now on your spatial location (ie you're in a projected crs) the above formula will give you square mapunits (km^2, m^2 whatever). In case you'...

11

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 ...

11

I think you'll find there is a bit of overlap with these definitions. They're all very similar, in my opinion. However, ESRI has a glossary of GIS terms, so I just looked them up. The definitions are similar or identical to the wiki GIS glossary as well. Bounding Box (Bounding Rectangle): [map display] The rectangle, aligned with the coordinate axes and ...

11

They're mostly from the EPSG - https://www.epsg-registry.org/ To be unique, you have to specify where they are from (basically a namespace - EPSG:4326, not just 4326). However most people will interpret them as EPSG if not specified. Originally, the EPSG registry was maintained in a Microsoft Access database and the well-known IDs (WKIDs) were assigned by ...

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