The Criminal Code requires that an arresting officer subjectively have reasonable grounds on which to base an arrest.
But more is required. In addition, the grounds must be justifiable from an objective point of view. To say the same thing in
another way, a reasonable person in the position of the officer
must be able to conclude that there were indeed reasonable
grounds for the arrest. On the other hand, nothing more
than reasonable grounds need be shown. Not a prima facie case.
And not proof beyond a reasonable doubt: Storrey, at pp. 250-51
 Possession of marijuana is only an indictable offence
where the amount in a person’s possession exceeds 30 grams,
based on the totality of the circumstances, including but not limited to the smell of marijuana in the vehicle: R. v. Loewen, 
2 S.C.R. 167,  S.C.J. No. 100, 2011 SCC 21, at para. 4.
 No bright line rule prohibits the presence of the smell of
marijuana as the source of reasonable grounds for an arrest.
However, what is dispositive are the circumstances under which
the olfactory observation was made. Sometimes, police officers
can convince a trial judge that their training and experience is
sufficient to yield a reliable opinion of present possession. As
with any item of evidence, it is for the trial judge to determine
the value and effect of the evidence: R. v. Polashek (1999),
45 O.R. (3d) 434,  O.J. No. 968, 134 C.C.C. (3d) 187 (C.A.),
at para. 14; R. v. Morris,  O.J. No. 1583, 2013 ONCA 223,
106 W.C.B. (2d) 279, at para. 8; R. v. Hoang,  O.J. No.
2922, 2013 ONCA 430, 107 W.C.B. (2d) 661, at para. 5.
 It is beyond controversy that a search incident to arrest
has its genesis in a lawful arrest. No further showing of reasonable grounds for the search itself is required: Cloutier v.
Langlois,  1 S.C.R. 158,  S.C.J. No. 10, at p. 185-86
S.C.R. However, the search must be truly incidental to the
arrest. There must be some reasonable basis for the search, for
example, to ensure the safety of the public and police; to protect
evidence from destruction; or to discover evidence. To be truly
incidental to the arrest means that the police must be attempting to achieve some valid purpose connected to the arrest. This
involves both subjective and objective elements. The police must
have one of the purposes for a valid search incident to arrest in
mind when conducting the search. And the searching officer’s
belief that this purpose will be served by the search must be reasonable: R. v. Caslake,  1 S.C.R. 51,  S.C.J. No. 3,
at paras. 19, 20 and 25.
 Where the justification for a search incident to arrest is
to find evidence, there must be some reasonable prospect of