CartesianObjLength vs ObjectLength Functions?

[map1]

Hello,

Can someone explain the difference between these two functions?

I was also wondering if someone could point me in the right direction
for descriptions of what each function type does?

Thanks for any help!

[Driver, Greg 9434]

ObjectLen( ):
The ObjectLen( ) function returns the length of an object expression. Note that only line and polyline objects have length values greater than zero; to measure the circumference of a rectangle, ellipse, or region, use the Perimeter( ) function.

The ObjectLen( ) function returns a length measurement in the units specified by the unit_name parameter; for example, to obtain a length in miles, specify "mi" as the unit_name parameter. See Set Distance Units statement for the list of valid unit names.

For the most part, MapInfo Professional performs a Cartesian or Spherical operation. Generally, a Spherical operation is performed unless the coordinate system is nonEarth, in which case, a Cartesian operation is performed.

CartesianObjectLen( ):
The CartesianObjectLen( ) function returns the length of an object expression. Note that only line and polyline objects have length values greater than zero; to measure the circumference of a rectangle, ellipse, or region, use the Perimeter( ) function.

The CartesianObjectLen( ) function will always return a value using a cartesian algorithm. A value of -1 will be returned for data that is in a Latitude/Longitude coordinate system, since Latitude/Longitude data is not projected and not cartesian.

The CartesianObjectLen( ) function returns a length measurement in the units specified by the unit_name parameter; for example, to obtain a length in miles, specify "mi" as the unit_name parameter. See the Set Distance Units statement for the list of valid unit names.

HTH

Greg Driver

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Hello,

Thanks for any help!

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[map1]

Thanks Greg. Both explanations appear the same to me.

However the reason I ask is that I've done a test with three different
polylines and updated attribute fields with both ObjectLen and
CartesianObjLength for each polyline.

I get the following results

Polyline 1

ObjLength (m): 5,712.87
CartesianObjLenght(m): 5,729.17

Polyline 2

ObjLength (m): 3,045.30
CartesianObjLenght(m): 3,053.98

Polyline 3

ObjLength (m): 1,664.29
CartesianObjLenght(m): 1,669.03

Can someone explain why I'm getting the different lengths? Obviously
this has an impact on tenders, works, etc so i need to know which is
the most accurate measurement to use.

[Tony Pilkington]

All non cartesian distance calculations in MI are based a sphere (Spheroid
12 radius 6370997.0m e= 0.0); the clue is in the name. You will find that
sphericaldistance() between two points is the same no matter what ellipsoid
you are using. This means that all distances below 48 degrees are short and
above that long. If you try to use the wrong distance function you get a
distance of -1, indicating an error.

This spheroid is worth remembering as many organisations publish rectified
imagery or radar data etc based on this sphere; 10,999,12,0,0,0 is Mercator
on this Sphere.

The error in the distance calculation is sometimes deemed to be acceptable
as it is about 1km in 500km; would you use a distance measuring device which
gave you an error or 1m in 500m?

Polyline 1

Polyline 2

Polyline 3

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[map1]

Interesting. Do you know which object length function would be best to
use when using British National Grid?

[CDR Group]

The British National Grid is a Cartesian Grid.

And you could also set your defaults to Cartesian if you are always using BNG.

 

Regards

 

John Ievers

CDR Group
www.cdrgroup.co.uk
www.browse-a-map.co.uk
Tel: 01433 621282
Fax: 01433 621292
Specialists in Geographic Information Systems

CDR Group is the trading name of Contract Data Research Ltd.
Registered address Eccles House, Eccles Lane, Hope, Hope Valley, S33 6RW.
Registration No. 1972326
VAT No. GB373 3117 67

----- Original Message -----

From: map1

To: MapInfo-L

Sent: Monday, November 07, 2011 2:26 PM

Subject: [MI-L] Re: CartesianObjLength vs ObjectLength Functions?

Interesting. Do you know which object length function would be best to
use when using British National Grid?

[map1]

Thankyou all for your help it makes alot more sense now.

I was wondering if anyone could point me in the right direction for
definitions of all the function in mapinfo?

[Peter Horsbøll Møller]

Have a look at the MapBasic Reference Guide - that should give you a good overview of the functions in MapInfo/MapBasic

You Can find it at pbinsight.com: http://www.pbinsight.com/support/product-documentation/details/mapinfo-mapbasic

Peter Horsbøll Møller
Pitney Bowes Business Insight

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Peter Horsbøll Møller
Pitney Bowes Business Insight - MapInfo

[Bill Thoen]

Check out PBBI MapInfo's support site:
http://www.pbinsight.com/support/product-documentation/details/mapinfo-professional

and download the manual(s).


Happy RTFM'ing!

- Bill Thoen