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How to get the accurate measurement result?

weiliang

Hi,

I am working on the measurement widget, and it seems that the out of box 'esri/dijit/Measurement' widget is always based on Web-Mercator projection. This makes the measurement result not as accurate as local projection if I understand correctly. Is there a way to use local projection to do the measurement by using this widget? Or I must build a fresh new widget from scratch by myself if I want to get accurate measurement result?

Thanks,

Wei

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MelitaKennedy

Hi Wei,

I don't use the Javascript API, but what about esri.geometry.geodesicUtils?

Melita

JonathanUihlein

Hi,

I am working on the measurement widget, and it seems that the out of box 'esri/dijit/Measurement' widget is always based on Web-Mercator projection. This makes the measurement result not as accurate as local projection if I understand correctly. Is there a way to use local projection to do the measurement by using this widget? Or I must build a fresh new widget from scratch by myself if I want to get accurate measurement result?

Thanks,

Wei

You only really need the conversion algorithm for Web-Mercator to your local projection.

With this, you could modify the widget to fit your needs or convert the values after a measurement event.

weiliang

You only really need the conversion algorithm for Web-Mercator to your local projection.

With this, you could modify the widget to fit your needs or convert the values after a measurement event.

Thanks for your reply. But how to use the conversion algorithm? Do you mean convert the graphic layer from Web_Mercator to my local projection. I really get confused here. I think the MeasureWidget only accept WebMercator input.

Wei

weiliang

Hi Wei,

I don't use the Javascript API, but what about esri.geometry.geodesicUtils?

Melita

Thanks for your reply. But I don't think using geodesic calculation can get the accurate result.

Wei

JonathanUihlein

Ideally you would convert your resulting measurement data to another spatial reference.

As you said, the Measurement widget works with Web Mercator (102100) and geographic coordinate systems (4326).

AdrianMarsden

Sorry, but are you actually saying that the measure widget is inaccurate for all other projections? I hope not as this is now in my production system and being used daily. All my mapping is in BNG 27700

JohnGravois

adrian,
as per the online documentation, the measurement widget will use geometry service (if provided) to reproject input geometries prior to calculating measurements.

Measuring distances and areas when your map uses the Mercator projection
http://blogs.esri.com/esri/arcgis/2010/03/05/measuring-distances-and-areas-when-your-map-uses-the-mercator-projection/

JeffPace1

Just a side not, but my (occasionally not so humble) opinion:

The inherent inaccuracies of browsers, pixels, screen scaling, clicking, etc.. will all add more inaccuracies than the difference between working natively in the local coordinate system and the errors introduced in the web mercator to local conversion.

If you need something more accurate than that, you need a surveyor, not a web application.

JonathanUihlein

Sorry, but are you actually saying that the measure widget is inaccurate for all other projections? I hope not as this is now in my production system and being used daily. All my mapping is in BNG 27700

John and Jeff said it best.

On a related note: there will be some (frequently requested) enhancements to the Measurement Widget in the next major release.

JeffPace1

Sorry, but are you actually saying that the measure widget is inaccurate for all other projections? I hope not as this is now in my production system and being used daily. All my mapping is in BNG 27700

Any time you do a conversion errors are introduced.

If you have verified your measurements to make sure that you are working in and reporting measurements of the correct projection, then the measurement widget produces results as accurate was a web system can.

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