More

Measuring distance between points in ArcMap - what coordinate system to use?


This is a basic -newbie - question. I have some handwritten data points taken off a GPS - so I am assuming they are in WGS 84, and I want to load them in ArcMap 9.3, via excel, and measure the distance between them (they are transects). I would like to know how best to maintain accuracy, and what format the data the coordinates should be when I load them into ArcMap.

The coordinates look like: 39. 51 550, 148.16 841 but the way they have been written they could be 39. 51 .50, 148.16.841 or 39 51.550, 148 16.841. Do these look like WGS 84 coordinates and what is the best way to input them into excel? Do I have to set or reset the projection in ArcMap before I take the measurements?

As you can see I am not confident with the basics of the coordinate system and projection system so any help and tips would be appreciated.


The coordinates look like degrees decimal-minutes. You'll need to convert them to decimal degrees to import them as WGS84. Do this in excel and then add the table to ArcGIS as an Excel table. Then create an XY Event layer from the coordinates. Finally, convert this to a geodatabase feature class of shapefile.

If you want to measure the distances between points on a one by one basis, your best bet is to use the measure tool's geodesic option. See this topic for more detail: http://webhelp.esri.com/arcgiSDEsktop/9.3/index.cfm?TopicName=Measuring_distances_and_areas

Craig


In addition to what @Craig Williams has suggested after you have your layer defined in the WGS84 coordinate system you should then re-project your layer to a Projected Coordinate System which is better for measuring purposes.

http://blogs.esri.com/Dev/blogs/arcgisserver/archive/2010/03/05/Measuring-distances-and-areas-when-your-map-uses-the-Mercator-projection.aspx


WGS 1984 Web Mercator and WGS 1984 Web Mercator (Auxiliary Sphere) use a conformal projection that preserves direction and the shape of data but distorts distance and area. Published in 1569 by Gerardus Mercator, the Mercator projection was created for use in navigation. A straight line drawn on a map in this projection provides a bearing by which one could fly a plane or sail a ship between two points.

Examine the illustrations below to view various map projections and their different properties. The maps display the continents plus a 15x15 degree graticule that covers the entire surface of the earth.

1) WGS 1984: The 15x15 degree graticule cells appear as squares. The map uses a modified Plate Carree projection to display the latitude and longitude values. Note that distances are very distorted in the east-west direction, north or south of the Equator. In this coordinate system, the North and South Poles appear as lines as long as the Equator. One degree at the Equator measures about 69.2 miles, or 111 kilometers on the ground, but a degree has a length of 0 at the North Pole and South Pole because the poles are points.

2) WGS 1984 Behrmann (World): This is an equal area projection. Note that the 15x15 degree graticule cells are now compressed to show the smaller areas north or south of the Equator. Distortion in the east-west direction is the same as that in GCS_WGS_1984 in #1 above.

3) WGS 1984 Aitoff: This compromise projection does a fairly good job of preserving both shape and area for the world. Notice that the poles in this projection are displayed as points rather than lines.

4) WGS 1984 Azimuthal Equidistant: In this projection, distances are correct when measured from the center of the projection. This is also a true direction projection.

5) WGS 1984 Web Mercator: This projection distorts data in the east-west direction, as do WGS 1984 and WGS 1984 Behrmann, but the worst distortion is in the north-south direction. Examine this image carefully, and note that in this projection Antarctica appears larger than the land masses of the other six continents combined.

For further information about map projections supported in ArcGIS Desktop, the properties of each projection, and the geographic areas for which each projection should be used, refer to the Projections Table linked to Knowledge Base article 24646, "Select a suitable map projection or coordinate system" at the link in the Related Information section below.


Measuring distance

I was wondering about this statement:
"You want to make accurate measurements from your map and be sure that the spatial analysis options you use in ArcMap calculate distance correctly. Latitude-longitude is a good system for storing spatial data, but not as good for viewing, querying, or analyzing maps. Degrees of latitude and longitude are not consistent units of measure for area, shape, distance, and direction. "

As I understand this that mean that I can not use a geographic coordinate system to measure correct distance. However when choosing Geographic coordinate system -> World -> WGS 1984 I get the distance between London and Los Angeles to be around 8800 km, which is correct according to my GIS textbook. I also tried measuring a street in Norway to look how it turned out in a small scale. This was also the approximately correct measurement.

However using different projected coordinate systems (e.g. Mercator) I got completely wrong distances.

I dont really understand this? I would guess all projected coordinate systems would have errors when measuring large scale distance since it is a flat projection? Shouldn't geographical coordinate systems be better for measuring distance as this is based on a spherical world?

If I want to make polygons of an area in one UTM zone and compare the size of it visually by moving it into another polygon in another zone how do I do this without getting error? If I draw a polygon in one UTM zone (lets say UTM zone 45) using UTM 45 as the projection and move it to the reference zone (utm zone 29) will I not get error when changing the utm zone to 29? How do I best solve this issue? I tried to do it with a polygon in zone 33 and measured it. Then I moved it to 29 and then I got a measurement error compared to when I measured the polygon in zone 33. Dont know if this is even possible to solve?


Keyboard shortcuts to enhance measuring tasks

To improve the experience when measuring in 3D, certain keyboard keys have been enabled to provide added function.

Press and hold the spacebar when you want to navigate the view to change your perspective in the middle of a measure sketch. You can do this midsketch, or you can pause ( ESC ) the sketch first and save the last point digitized before navigating.

Pause the sketch. This is useful before pressing the SPACE keyboard shortcut to navigate.

Clear & reset the sketch and results. This is the same as clicking the reset button on the Measure window.

Resume the sketch if you have paused it pressing the ESC keyboard shortcut.

If you have finished your sketch ( double-click to end), pressing the TAB keyboard shortuct will also resume the sketch as long as you have not changed to another tool. It will continue from the last digitized vertex remembered.

If you have Show Total turned on, it will also resume after pressing the TAB keyboard shortcut.


Measuring distances and areas

The Measure tool lets you draw on the map to measure lines and areas. You can use this tool in several ways. For example, you can draw a line or polygon on the map and get its length or area, or you can even click directly on a feature and get measurement information.
When you click the Measure tool, the Measure window appears. This dialog box allows you to set different options for how you measure including whether to measure lines, areas, or features use snapping and set which units are reported. Measurements are displayed inside the window, so it is easy to copy and paste them into other applications.

The Measure window contains tools for measuring distance and features. By default, the distance (line) measurement tool is enabled until you choose a different option from the Measure window.

The tools on the Measure window are listed below:

  • Measure a line. Double-click to complete the line.
  • Measure an area. Double-click to complete the polygon. (This is disabled if your data frame is not using a projected coordinate system).
  • Click a feature to measure its length (line), perimeter, and area (polygon or annotation) or x,y location (point features).
  • Snap to features while measuring.
  • Keep a sum of consecutive measurements.
  • Set the distance and area measurement units. The measure units are set to the map units by default.
  • Clear the measurements.

Geodesic versus Cartesian measurements with the Measure tool

By default, if the data frame is using a projected coordinate system, 2D Cartesian mathematics is used to calculate distances. The measurement reflects the projection of the 3D data onto the 2D surface and does not take into account the curvature of the earth. You can hold down the Shift key while measuring to get the geodesic distance instead. This is calculated using the spheroid/ellipsoid of the data frame's projected coordinate system's geographic coordinate system.
If the data frame is using a geographic coordinate system and the display units are linear, the measurements are geodesic by default and you don't need to hold down Shift.


Scale bar inaccurate on a projected geo-referenced map image, why?

I want to geo-reference a series of map, digitize features using polygons, and calculate accurate area measurements. I encountered the following issue:

After georeferencing a map image (.png) using the lat-long posted on the four corners of the map and projecting it using an appropriate UTM projection (WGS_1984_UTM_Zone_29N), I noticed that the scale bar is not accurate (the 50 km scale bar measures 34.4 km on the projected map). However, using the Measure Tool to find the length (in km) of 1 degree of latitde and lengths of 3.5 degrees longitude along both 46 deg. & 47.5 deg. latitude the measurement results and corresponding errors are listed in the table below:

Note the large error from the scale bar (>30% error) and relatively insignificant error on the latitude and longitude measurements (<0.1% error).

1. Why would the scale bar length be inaccurate (especially when the image lat-long appear to be projected correctly and can be measured accurately in ArcMap)?

2. Is this an error in my geo-referencing? If so, is there a better method for geo-referencing this type of map image?

3. Am I using an incorrect projection? I believe a projected UTM is appropriate for area calculations and should provide accurate length measurement for features located within the UTM zone.


The map coordinate system

As the Earth is round, when positions on the Earth are to be established, those positions must be transferred from the real-world locations to the map via a coordinate system. The map coordinate system is a reference of global positions on a flat map. The origins of coordinates are on the Earth’s surface. It is a rectangular coordinate system originated from intersection of at least two axes. There are two types of coordinate systems, which are two-dimensional and three-dimensional coordinate systems. These coordinates are references of the global positions with geographic coordinate systems.

1. Geographic coordinate systems

The Geographic coordinate systems establish positions on the Earth by referencing the longitude and latitude values measured from the angular distances from the origins of latitude and longitude. The origin of latitude is established from the point where it intersects the Earth’s center and is perpendicular to the rotation axis. That originating plane is called an equator which divides the globe into northern and southern hemispheres. Latitude values are measured relatively to the equator and range from –90 degrees at the South Pole to +90 degrees at the North Pole. As such, the reference of latitude values to indicate positions on the Earth will be measured by Degrees Minute Second, and marked with the letters to indicate North or South directions, such as Latitude 30 degrees 20 minutes 15 seconds North.

The origin of longitude, meanwhile, is established from the vertical plane in line with the globe’s axis where it passes the astronomical tower in Greenwich, UK. This origin is called the Prime Meridian which divides the Earth into the Eastern and Western hemispheres.

Longitude values are measured relatively to the prime meridian. They range from -180 degrees when travelling west to 180 degrees when travelling east. The measurement unit of longitude values are the same as that of the latitudes, except the direction’s marks which indicate the west or south azimuth, such as Longitude 90 degrees 20 minutes 45 seconds West.

2. UTM coordinate systems

UTM coordinate systems are adapted from Transverse Mercator map projection to maintain conformality property by using a cylinder to intersect the globe between Latitude 84 degrees north and 80 degrees south. The radius of a cylinder is shorter than that of the globe and the cylinder cuts through two meridian lines—inward and outward—which are called “Secant”, allowing more accuracy especially at both sides of the Central Meridian.

This type of coordinate systems were used by the US Army in 1946 to make maps with more accurate details. The systems are obtained from azimuthal and conformal map projections, and also come with standard regulations for worldwide application. Distances are measured by meters. Presently, the UTM coordinate systems are commonly used in both military and civil affairs. For Thailand, the Thai and US governments had agreed to make national maps in 1950 by using the Transverse Mercator projections, with UTM coordinate systems.

The global space between Latitude 80 degrees south and Latitude 84 degrees north is divided into 60 zones, each zone covering 6 degrees, on the longitudes. All the zones are numbered 1-60 respectively. Zone 1 is located between longitude 180 degrees west and 174 degrees west. Zone 2 is located next to Zone 1 on the east, followed by the remaining zones. Zone 60, the last one, lies between longitude 174 degrees east and 180 degrees east, adjacent to Zone 1. Each zone has its own central meridian. For example, Zone 1, between longitude 180-174 degrees west, will get the longitude 177 degrees west as its central meridian. Such features are found in every zone.

The space in each zone is divided into squares by parallels of latitude. Each parallel spacing is angled at 8 degrees, starting from latitude 80 degrees south, continuing with the 8-degree intervals passing the equator up to latitude 72 degrees north. Then from latitude 72-84 degrees north, the space is divided into 20 squares, each angled at 12 degrees. These squared space is called “Grid zone”. There are totally 1,200 grid zones. Dividing the space with this method will create rectangular grids of 6 x 8 degrees, except the area between latitude 72-84 degrees north which has the grid size of 6 x 12 degrees. After the division, the Roman alphabet from C to X (except I and O) are written on the divided space, starting with letter C from latitude 80 degrees south.

The grid table is inscribed with numbers and alphabet which are called UTM Grid zone destination. The numbers are read right up. For example, “47 Q” refers to the 47th vertical zone and the horizontal zone Q. Letters A, B and Y, Z are used for the Universal Polar Stereographic in both polar regions.

Distances in UTM coordinate systems are measured by meters. In each zone, the the central meridian intersects the equator at the right angle. The intersection point is called the zone origin of UTM coordinate systems. The direction that is parallel to the central meridian and heading northward is called “Grid north”. The eastern coordinates are set with Easting 500,000 meters from the false origin, while the northern coordinates for the equator are plotted in two cases, including Northing 0 meter from the equator and Northing 10,000,000 meters from the false origin. Therefore, the coordinates of zone origin of UTM system are E 500,000 m N 0 m for the northern hemisphere, and E 500,000 m N 10,000,000 m for the southern hemisphere. In addition, the use of UTM coordinate values can overlap that of adjacent zones with 40-km distance for convenient use at the periphery of each zone.

3. ระบบพิกัดแผนที่ GLO (General Land Office grid system)

This is another type of coordinate systems that helps for the division of surveyed areas to make geographic maps. It is commonly used for reading and making geological maps. In this coordinate system, the space is partially divided with each part being defined as follows:

  • Base line and Township line Any referred latitude in the surveyed area is called “base line”, and the parallels above and below the base line in every 6-mile spacing are Township lines.
  • Principal meridian and Range line The longitude referred to in a survey is called the principal meridian. The point of intersection with a base line is called Initial point. The lines parallel to the principal meridian on the east and west in every 6-mile spacing is the range line.
  • Township area is a square space of 6 x 6 miles, rounded with township lines and range lines. This area of 36 square miles is established by using distant positions from the base line and principal meridian. For example, 2N., R.1W is on the second township line above the 1st base line and range line on the west of the principal meridian.
  • Section The 36- square mile area of the township line is divided into 36 squares, each square covering 1 square mile which is called “section”.
  • Quadrangle A geographic map which is divided in accordance with this system is normally in rectangular shape. The space of rectangular map is framed with longitudes in the east and west, and latitudes in the north and south. The rectangular map is named after important cities or distinguished geographical features in the map. The quadrangle maps used in the US are classified by the distance between surrounding longitudes and latitudes into four types, which are:

The map of 1 Degree series, using a 1:250,000 scale
The map of 30 Minute series, using a 1:125,000 scale
The map of 15 Minute series, using a 1:62,500 scale
The map of 7.5 Minute series, using a 1:24,000 scale

How to read coordinates from the GLO maps

- Find the number attached to the section where the place is located. For example, 21 is read Sec.21.
- Find the township position on the township line where the place is located from the left or right periphery of the map, such as T.1N.
- Find the Range position on the Range line where a place is located at the top or bottom of the map, such as R.2W.
- To identify positions within the section, the space must be divided to determine the azimuth where the subsection is located and the scale comparing subsection with the pre-divided space. These indicators are placed before the read position of the section, such as NE ¼ SW ¼ Sec.21 T.1N R.2W

ภาพแสดงการแบ่งพื้นที่ และการอ่านตำแหน่งสถานที่จากแผนที่ระบบพิกัด GLO

Military grid

Because finding position by the degree-minute-second measurement is difficult and slow, a new method was invented to be used in army affairs. This method is called “military grid” which is a rectangular coordinates system comprising a group of lines that are parallel and almost in the north-south direction, used for measuring distance on the east of the origin, as well as a group of parallels that are almost in the east-west direction and also intersecting and perpendicular to the first group of lines. This second group is used for measuring distance above the origin. The intersection of these two groups of lines create squares, which are printed on the map. They are called “grid squares”, which are displayed together with numbers written at the map peripheries, showing distance from the origin. Normally, the distance from the origin is represented with numbers only once at the left bottom of the map. Of those numbers indicating distance from the false origin of other lines, the last 3 or 4 digits of the full number are omitted. This also depends on the size of grid intervals. For instance, the L7017 and L7018 map series will have grid distance of 1,000 meter. Therefore, the last three digits that will be omitted are 000, which are shown in the military grid.

In fact, the UTM map projection and the military grid are not different. The military grid or grid coordinates are tools for reading maps with UTM projection. Both are relevant to each other. Military grid is the part that clearly depicts the positions referred to with UTM projection, making it better understood. Therefore, the geographic map created by the Royal Thai Survey Department has used military grid to identify positions on the map, as the system is easy to understand and also enables quick and efficient use of the map.
A tip for writing or reading grid values used in military affairs is to “read right up”. The lower left intersection point is the grid coordinate value of that particular square.
The geographic coordinates system is therefore important for determining positions on the map, to indicate the positions in the real-world geography. It is the system that every map user needs to understand and know the correct method to read it, so the map can be used practically and efficiently.


Find_transformation¶

The find_transformations function is performed on a geometry service resource. This function returns a list of applicable geographic transformations you should use when projecting geometries from the input spatial reference to the output spatial reference. The transformations are in JSON format and are returned in order of most applicable to least applicable. Recall that a geographic transformation is not needed when the input and output spatial references have the same underlying geographic coordinate systems. In this case, findTransformations returns an empty list. Every returned geographic transformation is a forward transformation meaning that it can be used as-is to project from the input spatial reference to the output spatial reference. In the case where a predefined transformation needs to be applied in the reverse direction, it is returned as a forward composite transformation containing one transformation and a transformForward element with a value of false.

Inputs: in_sr - The well-known ID (gis,WKID) of the spatial reference or a

spatial reference JSON object for the input geometries

out_sr - The well-known ID (gis,WKID) of the spatial reference or a

spatial reference JSON object for the input geometries

extent_of_interest - The bounding box of the area of interest

specified as a JSON envelope. If provided, the extent of interest is used to return the most applicable geographic transformations for the area. If a spatial reference is not included in the JSON envelope, the in_sr is used for the envelope.

num_of_results - The number of geographic transformations to

return. The default value is 1. If num_of_results has a value of -1, all applicable transformations are returned.

future - boolean. This operation determines if the job is run asynchronously or not.


The buffer function is performed on a geometry service resource The result of this function is buffered polygons at the specified distances for the input geometry array. Options are available to union buffers and to use geodesic distance.

geometries - The array of geometries to be buffered. in_sr - The well-known ID of the spatial reference or a spatial

reference JSON object for the input geometries.

unit - The units for calculating each buffer distance. If unit

is not specified, the units are derived from bufferSR. If bufferSR is not specified, the units are derived from in_sr.

out_sr - The well-known ID of the spatial reference or a

spatial reference JSON object for the input geometries.

buffer_sr - The well-known ID of the spatial reference or a

spatial reference JSON object for the input geometries.

union_results - If true, all geometries buffered at a given

distance are unioned into a single (gis,possibly multipart) polygon, and the unioned geometry is placed in the output array. The default is false

geodesic - Set geodesic to true to buffer the input geometries

using geodesic distance. Geodesic distance is the shortest path between two points along the ellipsoid of the earth. If geodesic is set to false, the 2D Euclidean distance is used to buffer the input geometries. The default value depends on the geometry type, unit and bufferSR.


Parameters

A feature class containing a distribution of features for which the standard distance will be calculated.

A polygon feature class that will contain a circle polygon for each input center. These circle polygons graphically portray the standard distance at each center point.

The size of output circles in standard deviations. The default circle size is 1 valid choices are 1, 2, or 3 standard deviations.

  • 1 standard deviation — 1 standard deviation
  • 2 standard deviations — 2 standard deviations
  • 3 standard deviations —3 standard deviations

The numeric field used to weight locations according to their relative importance.

Field used to group features for separate standard distance calculations. The case field can be of integer, date, or string type.

A feature class containing a distribution of features for which the standard distance will be calculated.

A polygon feature class that will contain a circle polygon for each input center. These circle polygons graphically portray the standard distance at each center point.

The size of output circles in standard deviations. The default circle size is 1 valid choices are 1, 2, or 3 standard deviations.

  • 1_STANDARD_DEVIATION — 1 standard deviation
  • 2_STANDARD_DEVIATIONS — 2 standard deviations
  • 3_STANDARD_DEVIATIONS — 3 standard deviations

The numeric field used to weight locations according to their relative importance.

Field used to group features for separate standard distance calculations. The case field can be of integer, date, or string type.

Code sample

The following Python window script demonstrates how to use the StandardDistance tool.

The following stand-alone Python script demonstrates how to use the StandardDistance tool.


Watch the video: Entfernung und Abstand im Koordinatensystem (October 2021).