The information connecting numerical coordinates to actual locations on the earth.

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2
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1answer
338 views

OpenLayers and custom Proj4 Projection

ok so I'm a newbee in using OpenLayers. I try to map some planetary data on my webMap server using OpenLayers. I defined a proj4 projection as follow: Proj4js.defs["MYDATUM"] = "+proj=longlat ...
125
votes
12answers
28k views

What's the difference between a projection and a datum?

What's the difference between a projection and a datum?
2
votes
1answer
21 views

Figuring out what coordinates supposedly using 'NAD31 projection' refer to

I'm working with a GeoJSON file that contains sets of coordinates associated with street corners in the city of Montreal, Canada. The spec sheet of the API delivering those GeoJSON specifies that the ...
0
votes
1answer
94 views

ArcGIS Conversion Error While Converting ED50 Datum to WGS84 Datum

With ArcGIS for Desktop, I tried a datum conversion of Transverse Mercator ED50 Datum to Transverse Mercator WGS84 Datum. Normally, the difference between two projections is approximately 180 meters ...
0
votes
0answers
27 views

Comparing datums with GDAL/OGR

I have a bit of code in Java that takes in a SpatialReference object and checks that it is one of the acceptable datums (WGS84, NAD83, NAD83 HARN, NAD27), but because of the lack of standardization in ...
0
votes
1answer
39 views

Find AB_CSRS.gsb for NAD_1983_TO_WGS_1984_8 Transformation

I am trying to change my WGS84 data to NAD83 for Alberta data. My research tells me that I need to use the NAD_1983_TO_WGS_1984_8 Datum Transformation which is in the AB_CSRS.gsb. I can not find this ...
2
votes
1answer
57 views

Origin of the OSGB36 datum origin

Interpreting 'GIS' in the geographical information science sense, I have a historical question about the 1936 OSGB datum following the 1936-62 retriangulation of Great Britain - hopefully not too ...
0
votes
1answer
21 views

NAD conversion in Mexixo

I am looking for the file "Mexico.gsb" which used with ArcGIS when datum shifting from NAD27 to NAD83 in Mexico.
3
votes
1answer
121 views

Why not measure elevation from the center of the earth?

In trying to understand Geoid vs Ellipsoid height, I ran across this page. One of the claims is: Zero elevation as defined by Spain is not the same zero elevation defined by Canada ...
0
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0answers
31 views

WGS 84 Transformation issues with ArcGIS and Global Mapper

I have rasters, need to be geo-referenced in WGS 84 datum with Lat/Log (degree). I have geo-referenced using Global Mapper, which we usually used to use, because of its user friendly. Got a vector ...
0
votes
1answer
70 views

Persistent datum conflict error. Datums are the same

I'm clipping a number of features using other feature layers, and was careful to get everything into WGS84 beforehand, using what I thought were the proper datum transformations. Now I keep getting ...
2
votes
2answers
233 views

Older British Geological Survey geology maps of Kenya; need complete projection information

I have several geologic maps of Kenya produced by the BGS circa 1993. Noted on each map is the following: Projection:-Transverse Mercator Spheriod:-Clarke 1880 (modified) That's great, but that ...
0
votes
0answers
105 views

Myanmar Datum 2000 information

Does anybody know where I can find the technical details for Burma's latest datum? It's called Myanmar Datum 2000 and I can't find any details online. I need the details to create a custom datum on my ...
3
votes
2answers
53 views

What is the closest CRS to “Meridiano de Greenwich”

I have a 1906 scanned map that I am trying to georeference in QGIS for Mac. The map has French and English lat-long, indicating them as either "Meridiano de Greenwich" or "Meridiano de Paris." I would ...
2
votes
1answer
153 views

Geographic Transformations in Alaska Between NAD83 and WGS84

I have done a ton of research on the most accurate transformation to use in mainland Alaska to convert between the NAD83 and WGS84 datums, but I just can't seem to find a definitive answer. ArcMap ...
1
vote
1answer
137 views

Modify CRS to match boundaries of shapefile

I have boundary data in WGS'84 lat/long and data in a custom generated CRS. The data in the generated CRS appears to be offset by a particular vector compared to WGS'84. I want to attempt to match the ...
1
vote
1answer
43 views

OpenLayers Projection and Datum Question

I had a conversation with a co worker that was asking about an OL map I am developing. She asked me what the 'projection' was and I said EPSG:4326. She didn't know what that means, so I said it's ...
0
votes
0answers
31 views

Are ellipsoid and datum the same? [duplicate]

Are the datum and the ellipsoid the same? Please help me. What is a Datum? The Earth is shaped like a flattened sphere. This shape is called an ellipsoid. A datum is a model of the earth that is ...
5
votes
2answers
337 views

How to convert from one ellipsoid to another?

Introduction I have the following problem: I want to convert from WGS84 to "Anguilla 1957 / British West Indies Grid" (EPSG 2000). This seems to be straight-forward, as the transformation is a simple ...
1
vote
1answer
155 views

datum conversion with GDAL

I run gdaltransform like so: gdaltransform -s_srs nad27 -t_srs wgs84 -104.5 39.5 -104.5 39.5 0 gdaltransform -s_srs wgs84 -t_srs nad83 -104.5 39.5 -104.5 39.5 0 gdaltransform -s_srs nad27 ...
2
votes
3answers
361 views

Datum transformations, do we use wgs84 as an intermediate common reference?

I was transforming some data the other day using the 7 param helmert transformation and it got me thinking. I have a set of transformation parameters, but these only describe the transformation from ...
-1
votes
1answer
67 views

Mapinfo and qgis file's datum problem [closed]

please refer to following link for the explanation of the problem I had. do you have any idea on this? ...
2
votes
1answer
472 views

What is the process to calculate Helmert parameters?

I have some double points (the same points in 2 different datums). I'd like to calculate the Helmert parameters but I don't understand how. NOTE: I don't need software. I need an algorithm in order ...
3
votes
2answers
566 views

Where is list of Geographic Transformations in ArcGIS, and their areas of applicability?

I have some data in Provisional South American Datum 1956, and I need to project it to Web Mercator with WGS84 as the datum. When I use the project tool in ArcGIS desktop, it requires a Geographic ...
1
vote
1answer
102 views

Aligning layers with questionable metadata in SW Ontario

After making my best efforts to read/try everything I can on this forum and elsewhere, I unfortunately have to contribute to the wide literature of "my layers aren't aligning" questions. I am working ...
0
votes
3answers
167 views

Why does my map look stretched out?

I've decided to give this map design thing a go - I downloaded the USA counties shapefile from National Atlas, exported them to SQLite from QGIS, and then linked the SQLite file in Tilemill, and this ...
2
votes
1answer
99 views

What do different realizations of the same datum have in common?

Firstly, am I correct in assuming that the process of determining the shape and the size of the spheroid is independent from the process of determining the origin and the orientation? For example, ...
4
votes
2answers
79 views

towgs Definition for European Datum 1950

I am new to all things GIS so apologies for a basic questions. I am trying to specify the CRS of a climatic raster layer in R. It is an equal area projection based on Europe Equidistant Conic and the ...
25
votes
2answers
2k views

How accurate is approximating the Earth as a sphere?

What level of error do I encounter when approximating the earth as a sphere? Specifically, when dealing with the location of points and, for example, the great circle distances between them. Are ...
42
votes
7answers
3k 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 ...
3
votes
3answers
376 views

“Horizontal” and vertical datums terminology

I have heard people referring to as a horizontal datum to models such as WGS84 and as a vertical datum to geoids. The former is primarily used for determining latitude and longitude but also the ...
1
vote
0answers
127 views

Incorrect coordinates when reprojecting NAD27 to NAD83

I have some historic UTM coordinates as NAD27 (556787, 7169250) which I entered into a spreadsheet and have imported into QGIS. I assign them the projection Nad27 / UTM zone 12N (EPSG: 26712) I ...
0
votes
1answer
67 views

Transform from PSA56 to NAD83

I need to transform some data from PSA56 to NAD83. It doesn't have to be that rigorous as it's purely for a context map. I'm guessing I will need to go to WGS84 and then to NAD83. If anyone has any ...
1
vote
2answers
2k views

What do I need to change to get scale bar units other than degrees?

Measurements done in QGIS on my project are done properly using whatever unit is set in: Settings > Options > Map Tools > Preferred measurements units but the scale bar decoration and scale bar in ...
11
votes
2answers
367 views

Huge differences between results for 7-param datum transform

I'm trying to transform a WGS84 lat/lon point 11d42'32.10629" E 5d12'56.75371" S in three different software packages (Proj4, GeoTrans and Leica GeoOffice) but I receive very big differences ...
2
votes
1answer
28 views

IL East 1201 coordinate system

What is the difference between NAD CORS96 (Illinois East 1201) and the "normal NAD1983? We have data that was GPS'ed in CORS96, but data was imported into ArcMap using the "normal" NAD1983? What ...
0
votes
1answer
82 views

RFE raster images datum conflict

Im using ArcGIS 9.3. I've downloaded RFE images from FEWSNET. And im trying to load them on ArcMap, to clip them using Mozambique's shape. But as the RFE images related to Southern Africa come with ...
5
votes
1answer
100 views

Is there a way to determine the datum used when all you have is a lon and lat?

I have a dataset of US intersections from 2011 that I'm going to be using in a project together with data based in WGS84. Included in the CSV file is a variety of fields two of which are the lat and ...
1
vote
1answer
355 views

How do you convert from GPS data in WGS84 to a GIS built in NAD27 in ArcGIS?

I have GPS data that has been recorded in Datum WGS84. The client has a GIS built in Datum NAD27. Having looked into the conversions online I would like to know how to do this rigorously with ...
3
votes
2answers
210 views

GeometryServer > Project with Datum Transformation

I am facing the problem that when i use the extent in the url and i go from Amersfoort(WKID 4289) to WGS84(WKID 4326) that without the tranformation the point from longitude and latitude is placed ...
3
votes
3answers
690 views

Vertical Datum Conversion Utilities

I have some data (points and rasters) that I would like to be able to convert to and from several vertical datums, including MLLW in coastal Florida. I would appreciate it if people could provide some ...
1
vote
1answer
859 views

How to adjust online basemap to Arcmap

How can I adjust the online basemap imagery file to our existing ArcMap data. Below are steps I perform to add the basemap data. I Added basemap from "Add Data From GIS Online" in the file menu and ...
0
votes
0answers
133 views

How to find out what projection my data is using?

I have a set of data that I created from gps locations of images. The locations are in decimal. I don't know what projection was used when the data was captured. I used QGIS to display the data with a ...
1
vote
1answer
63 views

looking for datum to georeference maps of 19th century Maine

I am trying to georeference 3 (1840, 1844, and 1877) northern Maine maps. I really am not able to find which datum to use. Can anyone point me to a list of historic and regional datums? thanks
1
vote
1answer
36 views

Datums in compound CRSs

Is there some specific constraint regarding vertical and geodetic datums within a compound Coordinate Reference System (CRS)? E.g. if the ellipsoid model should be the same or not, or if the vertical ...
3
votes
0answers
332 views

How to set GDAL/cs2cs/proj to use NTv2 grid shift file when using EPSG codes?

I am using the GDAL API for Python for a script that takes only EPSG codes for inputs to specify source and target coordinate systems. While the cs2cs command seems to handle quite well ...
5
votes
3answers
269 views

How to manually change from GPS (WGS84) to web based maps projection systems?

I have collected location point data and wish to display this on a web based map. It was collected using a Garmin Etrex GPS using the WGS84 datum. I wish to display the points using Ushahidi and have ...
1
vote
2answers
216 views

define a datum and meridian in a CRS in QGIS

I have a shapefile defined by the source as: "The layer is in geographical (lat-long) coordinates (GCS_Clarke_1866; Datum: D_Clarke_1866; Prime Meridian: 0)." opening the shapefile in QGIS 1.8 the ...
6
votes
2answers
4k views

What datum (reference ellipsoid) does Google Earth use?

What datum (reference ellipsoid) does Google Earth[1] use? I can't seem to find where Google Earth tells explicitly what datum they use. I need some sort of valid link. [1] The default free version ...
9
votes
2answers
296 views

How do I perform a 3 parameter geotransformation and projection in Sql Server 2012?

I have a table with latitude longitude (NAD27) columns in it. I compute two other columns, X and Y, representing Web Mercator (WGS84) location. Currently I'm using a Arcmap to do this, by applying ...