the return number of LiDAR data set

I down load the LiDAR Point Cloud Datasets, for the 'return number' column. it is always either be '1' or '2',

as this:
X Y Z date time return number no returns off nidar ...

so does '1' stand for the 'first return ' and '2' stand for the 'last return'. or they have other meanings?

thanks

Jing

Good question. I believe you are asking about either the NSAF or Rainier datasets - which were both acquired by the same vendor and have these attributes.

The metadata document for these data (it can be downloaded under "Resources" > "Metadata & Files") describes these attributes as follows:

Up to four returns can be recorded per laser pulse; number_of_returns is the total returns for a pulse (up to a maximum of 4). Return_number is assigned as a number from 1 to 7 in a scheme that identifies which return is the last return recorded for a pulse:
1 first return with subsequent returns detected
2 second return with subsequent returns detected
3 third return with subsequent returns detected
4 fourth return
5 first return with no subsequent returns detected
6 second return with no subsequent returns detected
7 third return with no subsequent returns detected

Hope this is helpful.

-cc

yes, I was asking about the NSAF datasets.

thanks for your answer, that helps a lot *^_^*

sorry, I made something wrong last time, in that NSFA datasets I downloaded,the return number actually has three different values :1,2 and 3,

so as the explanation quoted below, if 3 stands for ' third return with subsequent returns detected ', then is there should also be '4' existed in that datasets then 4 could stands for 'the last return'?

also , after separating all the points has '1 from '2' and '3' into three groups, I found the points number in each group is quiet different, and none of these points in one group has the same longitude/altitude with others. But if these returns come from a same laser impulse, should each group has similar point number and each point has the same longitude/ altitude with point in other group.

So in this condition, how to identify the 'first return ' and 'last return'?

thanks

ccrosby wrote:

Good question. I believe you are asking about either the NSAF or Rainier datasets - which were both acquired by the same vendor and have these attributes.

The metadata document for these data (it can be downloaded under "Resources" > "Metadata & Files") describes these attributes as follows:

Up to four returns can be recorded per laser pulse; number_of_returns is the total returns for a pulse (up to a maximum of 4). Return_number is assigned as a number from 1 to 7 in a scheme that identifies which return is the last return recorded for a pulse:
1 first return with subsequent returns detected
2 second return with subsequent returns detected
3 third return with subsequent returns detected
4 fourth return
5 first return with no subsequent returns detected
6 second return with no subsequent returns detected
7 third return with no subsequent returns detected

Hope this is helpful.

-cc

Sorry for the delayed reply - I was on vacation last week.

Regarding the Return_Number values, I agree that something is odd if you are not seeing last returns ("4") in the data you've downloaded. I've never fully understood the attribute scheme used by Terapoint (the vendor who acquired these data)...I'll see if I can get clarification from someone on what to make of what you are seeing. I presume that you are downloading the full point cloud and not just the "Ground" classified points?

You should not expect each set of returns to have the same x & y position since the laser scanner is only perfectly vertical (at nadir) once per scan. For the rest of the scan the laser is pointing off nadir and hence is passing obliquely through the vegetation and will result in different x,y positions for returns from the same outgoing pulse. Also keep in mind that the point cloud is composed of multiple passes of the plane over a survey area and therefore when you download points from OpenTopo, you are getting a merged point cloud back that represents all returns for the area you selected.

-cc

None of the data hosted in OpenTopography are in geographic coordinates. In the case of the the NSAF and Rainier datasets, coordinates are in State Plane (CA and WA) - please see the metadata document for full coordinate system description. It is quite uncommon for LiDAR to be collected or analyzed in geographic coordinates, but if you want to convert these data to lat/long, you could use PROJ4 (http://trac.osgeo.org/proj/) to do so.

noirasura wrote:

here,are X,Y Z stand for 'latitude'/'longitude'/'elevation', if not, how to convert them to lat/long?